Add diff tracking — compare exports between runs

- New cantaloupe/diff.py: diff engine comparing xlsx by Asset ID,
  manifest tracking at ~/.cantaloupe-exports/manifest.json
- New  subcommand: python -m cantaloupe diff [--from FILE] [--to FILE]
- Export --diff flag: auto-compare with previous export after download
- Every export auto-records to manifest for history tracking
- 44 new tests, 130/130 pass total

cantaloupe/cli.py

@@ -3,11 +3,15 @@ Click CLI for cantaloupe export tool.
 
 Commands:
     export      Download machine list Excel from mycantaloupe.com
+    diff        Compare two machine list exports and show what changed
     configure   Save credentials to ~/.cantaloupe.env
 
 Usage:
     python -m cantaloupe export --output ./exports/
     python -m cantaloupe export --scheduled --output /data/exports/
+    python -m cantaloupe export --diff   # auto-compare with previous export
+    python -m cantaloupe diff --from old.xlsx --to new.xlsx
+    python -m cantaloupe diff            # compare two most recent exports
     python -m cantaloupe configure
 """
 
@@ -16,6 +20,7 @@ import os
 import sys
 import time
 from pathlib import Path
+from typing import Optional
 
 import click
 
@@ -83,6 +88,54 @@ def _resolve_output(output: str | None, scheduled: bool) -> Path:
     return out_path
 
 
+def _print_diff_result(result: dict, from_label: str, to_label: str) -> None:
+    """Pretty-print a diff result to the console."""
+    click.echo()
+    click.echo("═" * 60)
+    click.echo("  Diff Report")
+    click.echo("═" * 60)
+    click.echo(f"  From : {from_label} ({result['from_count']} rows)")
+    click.echo(f"  To   : {to_label} ({result['to_count']} rows)")
+    click.echo(f"  Key  : {result['id_column']}")
+    click.echo()
+
+    added = result["added_count"]
+    removed = result["removed_count"]
+    changed = result["changed_count"]
+
+    if added == 0 and removed == 0 and changed == 0:
+        click.echo("  ✅ No changes — exports are identical.")
+    else:
+        if added:
+            click.echo(f"  🟢 {added} machine(s) added")
+            for row in result["added"][:10]:
+                rid = row.get(result["id_column"], "?")
+                click.echo(f"       + {rid}")
+            if added > 10:
+                click.echo(f"       ... and {added - 10} more")
+
+        if removed:
+            click.echo(f"  🔴 {removed} machine(s) removed")
+            for row in result["removed"][:10]:
+                rid = row.get(result["id_column"], "?")
+                click.echo(f"       - {rid}")
+            if removed > 10:
+                click.echo(f"       ... and {removed - 10} more")
+
+        if changed:
+            click.echo(f"  🟡 {changed} machine(s) changed")
+            for entry in result["changed"][:5]:
+                rid = entry["id"]
+                field_names = list(entry["changes"].keys())
+                click.echo(f"       ~ {rid}  ({', '.join(field_names[:5])})")
+                if len(field_names) > 5:
+                    click.echo(f"         +{len(field_names) - 5} more fields")
+            if changed > 5:
+                click.echo(f"       ... and {changed - 5} more")
+
+    click.echo("═" * 60)
+
+
 # ── CLI group ───────────────────────────────────────────────────────────────
 
 
@@ -137,6 +190,12 @@ def cli(ctx: click.Context, verbose: bool) -> None:
     is_flag=True,
     help="Non-interactive mode for cron usage — skips credential prompts, uses defaults.",
 )
+@click.option(
+    "--diff/--no-diff",
+    "diff_flag",
+    default=False,
+    help="Compare with the previous export and show changes after download.",
+)
 @click.option(
     "--email",
     default=None,
@@ -155,6 +214,7 @@ def export(
     output: str | None,
     headless: bool,
     scheduled: bool,
+    diff_flag: bool,
     email: str | None,
     password: str | None,
 ) -> None:
@@ -163,6 +223,8 @@ def export(
     Authenticates with your Cantaloupe account and downloads the
     VueMachineList ExcelExport file, reporting file size, row count,
     and elapsed time on completion.
+
+    Use --diff to auto-compare with the previous export after download.
     """
     # Resolve credentials (CLI flags > env vars > config file)
     resolved_email = email or ctx.obj.get("email", "")
@@ -238,6 +300,32 @@ def export(
         click.echo(f"  Rows : {row_count:,}")
     click.echo(f"  Time : {elapsed:.1f}s")
 
+    # ── record to manifest ─────────────────────────────────────────────
+    from cantaloupe.diff import record_export
+
+    # Snapshot the previous export *before* recording this one, so
+    # we diff against the run-before-last, not this run.
+    previous_path: Path | None = None
+    if diff_flag:
+        from cantaloupe.diff import get_previous_export
+        previous_path = get_previous_export()
+
+    record_export(out_path, row_count, file_size)
+
+    # ── diff against previous export ────────────────────────────────────
+    if diff_flag:
+        if previous_path is None:
+            click.echo("\n⚠  No previous export found — skipping diff.")
+        elif previous_path.resolve() == out_path.resolve():
+            click.echo("\n⚠  Previous export is the same file — skipping diff.")
+        else:
+            try:
+                from cantaloupe.diff import diff_excels
+                result = diff_excels(previous_path, out_path)
+                _print_diff_result(result, previous_path.name, out_path.name)
+            except Exception as e:
+                click.echo(f"\n⚠  Diff failed: {e}", err=True)
+
 
 # ── configure command ───────────────────────────────────────────────────────
 
@@ -268,6 +356,86 @@ def configure(email: str, password: str) -> None:
     click.echo(f"  You can now run: python -m cantaloupe export")
 
 
+# ── diff command ─────────────────────────────────────────────────────────────
+
+
+@cli.command()
+@click.option(
+    "--from", "from_path",
+    default=None,
+    help="Path to the older xlsx file (default: second-most-recent export).",
+)
+@click.option(
+    "--to", "to_path",
+    default=None,
+    help="Path to the newer xlsx file (default: most recent export).",
+)
+@click.option(
+    "--id-column",
+    default=None,
+    help="Column name to use as unique machine identifier (auto-detected if omitted).",
+)
+@click.option(
+    "--json", "json_output",
+    is_flag=True,
+    help="Output the diff result as JSON instead of pretty-printed text.",
+)
+def diff(
+    from_path: str | None,
+    to_path: str | None,
+    id_column: str | None,
+    json_output: bool,
+) -> None:
+    """Compare two machine list Excel exports and show what changed.
+
+    If --from and --to are omitted, the two most recent exports are used
+    (tracked via ~/.cantaloupe-exports/manifest.json).
+
+    Reports machines added, removed, and changed — keyed on the Asset ID
+    column (auto-detected, or specified with --id-column).
+    """
+    from cantaloupe.diff import diff_excels, get_latest_exports
+
+    # Resolve file paths
+    if from_path and to_path:
+        f_path = Path(os.path.expanduser(from_path)).resolve()
+        t_path = Path(os.path.expanduser(to_path)).resolve()
+    elif from_path or to_path:
+        raise click.UsageError(
+            "Specify both --from and --to, or neither (to use latest exports)."
+        )
+    else:
+        latest = get_latest_exports(2)
+        if len(latest) < 2:
+            click.echo(
+                "ERROR: Need at least two exports to diff. "
+                "Run an export first, or specify --from and --to explicitly.",
+                err=True,
+            )
+            raise SystemExit(1)
+        f_path = latest[1]  # older
+        t_path = latest[0]  # newer
+
+    if not f_path.exists():
+        click.echo(f"ERROR: File not found: {f_path}", err=True)
+        raise SystemExit(1)
+    if not t_path.exists():
+        click.echo(f"ERROR: File not found: {t_path}", err=True)
+        raise SystemExit(1)
+
+    try:
+        result = diff_excels(f_path, t_path, id_column=id_column)
+    except (ValueError, FileNotFoundError) as e:
+        click.echo(f"ERROR: {e}", err=True)
+        raise SystemExit(1)
+
+    if json_output:
+        import json
+        click.echo(json.dumps(result, indent=2, default=str))
+    else:
+        _print_diff_result(result, f_path.name, t_path.name)
+
+
 # ── direct invocation ───────────────────────────────────────────────────────
 
 if __name__ == "__main__":

cantaloupe/diff.py

@@ -0,0 +1,238 @@
+"""
+Diff engine for comparing Cantaloupe machine list Excel exports.
+
+Reports:
+  - 🟢 New machines added (present in new, not in old)
+  - 🔴 Machines removed (present in old, not in new)
+  - 🟡 Machines with changed fields (same ID, different values)
+
+Also manages a JSON manifest at ~/.cantaloupe-exports/manifest.json
+that tracks export history for auto-diff functionality.
+"""
+
+import json
+import os
+from datetime import datetime
+from pathlib import Path
+from typing import Optional
+
+import openpyxl
+
+MANIFEST_DIR = Path.home() / ".cantaloupe-exports"
+MANIFEST_FILE = MANIFEST_DIR / "manifest.json"
+
+
+# ── xlsx reading ────────────────────────────────────────────────────────────
+
+
+def _read_xlsx(path: Path) -> tuple[list[str], list[dict]]:
+    """Read an xlsx file and return (headers, list of row dicts).
+
+    Each row dict maps header name → cell value.
+    Returns empty lists if the file is empty or unreadable.
+    """
+    wb = openpyxl.load_workbook(path, read_only=True)
+    try:
+        ws = wb.active
+        if ws is None:
+            return [], []
+        rows = list(ws.iter_rows(values_only=True))
+    finally:
+        wb.close()
+
+    if not rows:
+        return [], []
+
+    headers = [str(h) if h is not None else "" for h in rows[0]]
+    data = []
+    for row in rows[1:]:
+        row_dict = {}
+        for i, cell in enumerate(row):
+            if i < len(headers):
+                row_dict[headers[i]] = cell
+        data.append(row_dict)
+
+    return headers, data
+
+
+def _find_id_column(headers: list[str]) -> str:
+    """Find the best unique-identifier column from headers.
+
+    Priority:
+      1. Column whose name contains 'asset' AND 'id' (case-insensitive)
+      2. Column whose name equals 'id' (case-insensitive)
+      3. First column (fallback)
+    """
+    for h in headers:
+        hl = h.lower()
+        if "asset" in hl and "id" in hl:
+            return h
+    for h in headers:
+        if h.lower() == "id":
+            return h
+    return headers[0] if headers else ""
+
+
+# ── diff engine ─────────────────────────────────────────────────────────────
+
+
+def _normalize(val) -> str:
+    """Convert a cell value to a string for comparison and keying."""
+    if val is None:
+        return ""
+    return str(val).strip()
+
+
+def diff_excels(
+    from_path: Path,
+    to_path: Path,
+    id_column: Optional[str] = None,
+) -> dict:
+    """Compare two xlsx files and return a structured diff result.
+
+    Args:
+        from_path: Path to the older ("from") xlsx file.
+        to_path:   Path to the newer ("to") xlsx file.
+        id_column: Column name to use as the unique machine identifier.
+                   Auto-detected if not provided (looks for 'Asset ID').
+
+    Returns:
+        dict with keys:
+          - id_column: str               — column used as the key
+          - from_path, to_path: str      — file paths
+          - from_count, to_count: int    — data row counts
+          - added: list[dict]            — rows present in 'to' but not 'from'
+          - removed: list[dict]          — rows present in 'from' but not 'to'
+          - changed: list[dict]          — {id, changes: {col: {old, new}}}
+          - added_count, removed_count, changed_count: int
+
+    Raises:
+        ValueError: if either file has no data / no rows.
+        FileNotFoundError: if either file doesn't exist.
+    """
+    from_headers, from_data = _read_xlsx(from_path)
+    to_headers, to_data = _read_xlsx(to_path)
+
+    if not to_headers:
+        raise ValueError(f"No data found in {to_path}")
+
+    # Determine id column (prefer the 'to' file's headers)
+    id_col = id_column or _find_id_column(to_headers)
+
+    # Build lookup dicts keyed on the id column
+    from_by_id: dict[str, dict] = {}
+    for row in from_data:
+        key = _normalize(row.get(id_col, ""))
+        from_by_id[key] = row
+
+    to_by_id: dict[str, dict] = {}
+    for row in to_data:
+        key = _normalize(row.get(id_col, ""))
+        to_by_id[key] = row
+
+    from_ids = set(from_by_id.keys())
+    to_ids = set(to_by_id.keys())
+
+    added_ids = to_ids - from_ids
+    removed_ids = from_ids - to_ids
+    common_ids = from_ids & to_ids
+
+    # Collect changed rows
+    changed: list[dict] = []
+    # Use headers from 'to' file as canonical
+    for cid in sorted(common_ids):
+        old_row = from_by_id[cid]
+        new_row = to_by_id[cid]
+        field_changes: dict[str, dict[str, object]] = {}
+        for col in to_headers:
+            old_val = _normalize(old_row.get(col))
+            new_val = _normalize(new_row.get(col))
+            if old_val != new_val:
+                field_changes[col] = {"old": old_row.get(col), "new": new_row.get(col)}
+        if field_changes:
+            changed.append({"id": cid, "changes": field_changes})
+
+    return {
+        "id_column": id_col,
+        "from_path": str(from_path),
+        "to_path": str(to_path),
+        "from_count": len(from_data),
+        "to_count": len(to_data),
+        "added": [to_by_id[aid] for aid in sorted(added_ids)],
+        "removed": [from_by_id[rid] for rid in sorted(removed_ids)],
+        "changed": changed,
+        "added_count": len(added_ids),
+        "removed_count": len(removed_ids),
+        "changed_count": len(changed),
+    }
+
+
+# ── manifest management ─────────────────────────────────────────────────────
+
+
+def load_manifest() -> dict:
+    """Load the export history manifest from ~/.cantaloupe-exports/manifest.json.
+
+    Returns a dict with an 'exports' list.  Creates an empty manifest
+    if the file doesn't exist.
+    """
+    MANIFEST_DIR.mkdir(parents=True, exist_ok=True)
+    if MANIFEST_FILE.exists():
+        try:
+            return json.loads(MANIFEST_FILE.read_text())
+        except (json.JSONDecodeError, OSError):
+            return {"exports": []}
+    return {"exports": []}
+
+
+def save_manifest(manifest: dict) -> None:
+    """Save the manifest to disk."""
+    MANIFEST_DIR.mkdir(parents=True, exist_ok=True)
+    MANIFEST_FILE.write_text(json.dumps(manifest, indent=2, default=str) + "\n")
+
+
+def record_export(
+    filepath: Path,
+    row_count: int,
+    file_size: int,
+) -> None:
+    """Record a completed export in the manifest."""
+    manifest = load_manifest()
+    manifest.setdefault("exports", []).append(
+        {
+            "path": str(filepath),
+            "filename": filepath.name,
+            "timestamp": datetime.now().isoformat(),
+            "row_count": row_count,
+            "file_size": file_size,
+        }
+    )
+    save_manifest(manifest)
+
+
+def get_previous_export() -> Optional[Path]:
+    """Return the path of the most recent export, or None if no history."""
+    manifest = load_manifest()
+    exports = manifest.get("exports", [])
+    if not exports:
+        return None
+    previous = exports[-1]["path"]
+    p = Path(previous)
+    if p.exists():
+        return p
+    # File was moved/deleted — return None so we don't diff against a ghost
+    return None
+
+
+def get_latest_exports(n: int = 2) -> list[Path]:
+    """Return paths of the N most recent existing exports, newest first."""
+    manifest = load_manifest()
+    exports = manifest.get("exports", [])
+    result: list[Path] = []
+    for entry in reversed(exports):
+        p = Path(entry["path"])
+        if p.exists():
+            result.append(p)
+            if len(result) >= n:
+                break
+    return result

tests/test_diff.py

@@ -0,0 +1,772 @@
+"""
+Tests for cantaloupe diff engine and manifest tracking.
+
+Covers:
+  - diff.py: _read_xlsx, _find_id_column, diff_excels
+  - diff.py: load_manifest, save_manifest, record_export,
+             get_previous_export, get_latest_exports
+  - cli.py: diff command (--from/--to, --id-column, --json)
+  - cli.py: export --diff flag (auto-compare after download)
+"""
+
+import json
+import os
+import tempfile
+from pathlib import Path
+from unittest.mock import MagicMock, patch
+
+import openpyxl
+import pytest
+from click.testing import CliRunner
+
+from cantaloupe.cli import cli, _print_diff_result
+from cantaloupe.diff import (
+    MANIFEST_DIR,
+    MANIFEST_FILE,
+    _find_id_column,
+    _read_xlsx,
+    diff_excels,
+    get_latest_exports,
+    get_previous_export,
+    load_manifest,
+    record_export,
+    save_manifest,
+)
+
+
+# ── helpers for building test xlsx files ──────────────────────────────────
+
+
+def _make_xlsx(path: Path, headers: list[str], rows: list[list]) -> Path:
+    """Write a minimal xlsx file and return the path."""
+    wb = openpyxl.Workbook()
+    ws = wb.active
+    ws.append(headers)
+    for row in rows:
+        ws.append(row)
+    wb.save(path)
+    wb.close()
+    return path
+
+
+# ─────────────────────────────────────────────────────────────────
+# _read_xlsx
+# ─────────────────────────────────────────────────────────────────
+
+
+def test_read_xlsx_basic(tmp_path):
+    """Read a simple xlsx file with headers and data rows."""
+    path = _make_xlsx(
+        tmp_path / "test.xlsx",
+        headers=["Asset ID", "Name", "Location"],
+        rows=[
+            ["A001", "Machine 1", "Warehouse A"],
+            ["A002", "Machine 2", "Warehouse B"],
+        ],
+    )
+    headers, data = _read_xlsx(path)
+    assert headers == ["Asset ID", "Name", "Location"]
+    assert len(data) == 2
+    assert data[0] == {"Asset ID": "A001", "Name": "Machine 1", "Location": "Warehouse A"}
+    assert data[1] == {"Asset ID": "A002", "Name": "Machine 2", "Location": "Warehouse B"}
+
+
+def test_read_xlsx_empty(tmp_path):
+    """Empty xlsx (no rows) returns empty lists."""
+    path = _make_xlsx(
+        tmp_path / "empty.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[],
+    )
+    headers, data = _read_xlsx(path)
+    assert headers == ["Asset ID", "Name"]
+    assert data == []
+
+
+def test_read_xlsx_header_only(tmp_path):
+    """File with only a header row returns empty data list."""
+    wb = openpyxl.Workbook()
+    ws = wb.active
+    ws.append(["Asset ID", "Name"])
+    wb.save(tmp_path / "header_only.xlsx")
+    wb.close()
+
+    headers, data = _read_xlsx(tmp_path / "header_only.xlsx")
+    assert headers == ["Asset ID", "Name"]
+    assert data == []
+
+
+def test_read_xlsx_none_cells(tmp_path):
+    """None / blank cells are handled gracefully."""
+    path = _make_xlsx(
+        tmp_path / "blanks.xlsx",
+        headers=["Asset ID", "Name", "Notes"],
+        rows=[
+            ["A001", None, "some notes"],
+            [None, "No-ID Machine", ""],
+        ],
+    )
+    headers, data = _read_xlsx(path)
+    assert data[0]["Name"] is None
+    assert data[1]["Asset ID"] is None
+
+
+def test_read_xlsx_file_not_found(tmp_path):
+    """Missing file raises FileNotFoundError."""
+    with pytest.raises(FileNotFoundError):
+        _read_xlsx(tmp_path / "nonexistent.xlsx")
+
+
+# ─────────────────────────────────────────────────────────────────
+# _find_id_column
+# ─────────────────────────────────────────────────────────────────
+
+
+def test_find_id_column_exact_asset_id():
+    """Finds column with 'asset' and 'id' in name."""
+    headers = ["Name", "Location", "Asset ID", "Status"]
+    assert _find_id_column(headers) == "Asset ID"
+
+
+def test_find_id_column_case_insensitive():
+    """Case-insensitive match: 'asset id' matches 'Asset ID'."""
+    headers = ["Name", "asset id", "Location"]
+    assert _find_id_column(headers) == "asset id"
+
+
+def test_find_id_column_assetid_no_space():
+    """'AssetID' (no space) still matches."""
+    headers = ["Name", "AssetID", "Location"]
+    assert _find_id_column(headers) == "AssetID"
+
+
+def test_find_id_column_just_id():
+    """Falls back to column named 'id' (case-insensitive)."""
+    headers = ["Name", "Location", "id"]
+    assert _find_id_column(headers) == "id"
+
+
+def test_find_id_column_fallback_first():
+    """When no good match, returns the first column."""
+    headers = ["Name", "Location", "Status"]
+    assert _find_id_column(headers) == "Name"
+
+
+def test_find_id_column_empty_headers():
+    """Empty headers returns empty string."""
+    assert _find_id_column([]) == ""
+
+
+# ─────────────────────────────────────────────────────────────────
+# diff_excels
+# ─────────────────────────────────────────────────────────────────
+
+
+def test_diff_identical_files(tmp_path):
+    """Two identical files produce zero changes."""
+    old = _make_xlsx(tmp_path / "old.xlsx",
+        headers=["Asset ID", "Name", "Location"],
+        rows=[["A001", "Machine 1", "WH-A"], ["A002", "Machine 2", "WH-B"]],
+    )
+    new = _make_xlsx(tmp_path / "new.xlsx",
+        headers=["Asset ID", "Name", "Location"],
+        rows=[["A001", "Machine 1", "WH-A"], ["A002", "Machine 2", "WH-B"]],
+    )
+
+    result = diff_excels(old, new)
+    assert result["added_count"] == 0
+    assert result["removed_count"] == 0
+    assert result["changed_count"] == 0
+    assert result["from_count"] == 2
+    assert result["to_count"] == 2
+
+
+def test_diff_added_machines(tmp_path):
+    """New machines in 'to' file are reported as added."""
+    old = _make_xlsx(tmp_path / "old.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "Machine 1"]],
+    )
+    new = _make_xlsx(tmp_path / "new.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "Machine 1"], ["A002", "Machine 2"], ["A003", "Machine 3"]],
+    )
+
+    result = diff_excels(old, new)
+    assert result["added_count"] == 2
+    assert result["removed_count"] == 0
+    assert result["changed_count"] == 0
+    assert len(result["added"]) == 2
+    assert result["added"][0]["Asset ID"] == "A002"
+    assert result["added"][1]["Asset ID"] == "A003"
+
+
+def test_diff_removed_machines(tmp_path):
+    """Machines in 'from' but not 'to' are reported as removed."""
+    old = _make_xlsx(tmp_path / "old.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "Machine 1"], ["A002", "Machine 2"]],
+    )
+    new = _make_xlsx(tmp_path / "new.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "Machine 1"]],
+    )
+
+    result = diff_excels(old, new)
+    assert result["added_count"] == 0
+    assert result["removed_count"] == 1
+    assert result["changed_count"] == 0
+    assert result["removed"][0]["Asset ID"] == "A002"
+
+
+def test_diff_changed_fields(tmp_path):
+    """Same Asset ID with different field values is reported as changed."""
+    old = _make_xlsx(tmp_path / "old.xlsx",
+        headers=["Asset ID", "Name", "Location"],
+        rows=[["A001", "Machine 1", "WH-A"], ["A002", "Machine 2", "WH-B"]],
+    )
+    new = _make_xlsx(tmp_path / "new.xlsx",
+        headers=["Asset ID", "Name", "Location"],
+        rows=[["A001", "Machine 1", "WH-C"], ["A002", "Machine 2", "WH-B"]],
+    )
+
+    result = diff_excels(old, new)
+    assert result["added_count"] == 0
+    assert result["removed_count"] == 0
+    assert result["changed_count"] == 1
+    assert result["changed"][0]["id"] == "A001"
+    assert "Location" in result["changed"][0]["changes"]
+    assert result["changed"][0]["changes"]["Location"]["old"] == "WH-A"
+    assert result["changed"][0]["changes"]["Location"]["new"] == "WH-C"
+
+
+def test_diff_mixed_changes(tmp_path):
+    """Combination of added, removed, and changed machines."""
+    old = _make_xlsx(tmp_path / "old.xlsx",
+        headers=["Asset ID", "Name", "Location"],
+        rows=[
+            ["A001", "Machine 1", "WH-A"],
+            ["A002", "Machine 2", "WH-B"],  # will be removed
+            ["A003", "Machine 3", "WH-C"],  # will be changed
+        ],
+    )
+    new = _make_xlsx(tmp_path / "new.xlsx",
+        headers=["Asset ID", "Name", "Location"],
+        rows=[
+            ["A001", "Machine 1", "WH-A"],  # unchanged
+            ["A003", "Machine 3", "WH-D"],  # changed Location
+            ["A004", "Machine 4", "WH-E"],  # added
+        ],
+    )
+
+    result = diff_excels(old, new)
+    assert result["added_count"] == 1  # A004
+    assert result["removed_count"] == 1  # A002
+    assert result["changed_count"] == 1  # A003
+
+    assert result["added"][0]["Asset ID"] == "A004"
+    assert result["removed"][0]["Asset ID"] == "A002"
+    assert result["changed"][0]["id"] == "A003"
+
+
+def test_diff_explicit_id_column(tmp_path):
+    """Specify a custom id column."""
+    old = _make_xlsx(tmp_path / "old.xlsx",
+        headers=["Machine Number", "Name"],
+        rows=[["M001", "Machine 1"]],
+    )
+    new = _make_xlsx(tmp_path / "new.xlsx",
+        headers=["Machine Number", "Name"],
+        rows=[["M001", "Machine 1"], ["M002", "Machine 2"]],
+    )
+
+    result = diff_excels(old, new, id_column="Machine Number")
+    assert result["id_column"] == "Machine Number"
+    assert result["added_count"] == 1
+
+
+def test_diff_empty_to_file(tmp_path):
+    """Empty 'to' file raises ValueError."""
+    old = _make_xlsx(tmp_path / "old.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "Machine 1"]],
+    )
+    # Empty new file — no rows at all
+    wb = openpyxl.Workbook()
+    wb.save(tmp_path / "new.xlsx")
+    wb.close()
+
+    with pytest.raises(ValueError, match="No data found"):
+        diff_excels(old, tmp_path / "new.xlsx")
+
+
+def test_diff_empty_from_file_allows_all_added(tmp_path):
+    """Empty 'from' file means all 'to' rows are reported as added."""
+    wb = openpyxl.Workbook()
+    wb.save(tmp_path / "old.xlsx")
+    wb.close()
+
+    new = _make_xlsx(tmp_path / "new.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "Machine 1"], ["A002", "Machine 2"]],
+    )
+
+    result = diff_excels(tmp_path / "old.xlsx", new)
+    assert result["added_count"] == 2
+    assert result["removed_count"] == 0
+    assert result["changed_count"] == 0
+
+
+def test_diff_none_values_compared(tmp_path):
+    """None values are normalized to empty string for comparison."""
+    old = _make_xlsx(tmp_path / "old.xlsx",
+        headers=["Asset ID", "Notes"],
+        rows=[["A001", None]],
+    )
+    new = _make_xlsx(tmp_path / "new.xlsx",
+        headers=["Asset ID", "Notes"],
+        rows=[["A001", ""]],
+    )
+
+    result = diff_excels(old, new)
+    # None and "" are normalized to the same string → no change
+    assert result["changed_count"] == 0
+
+
+def test_diff_result_keys(tmp_path):
+    """Verify all expected keys are present in the result dict."""
+    old = _make_xlsx(tmp_path / "old.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "M1"]],
+    )
+    new = _make_xlsx(tmp_path / "new.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "M1"]],
+    )
+
+    result = diff_excels(old, new)
+    expected_keys = {
+        "id_column", "from_path", "to_path",
+        "from_count", "to_count",
+        "added", "removed", "changed",
+        "added_count", "removed_count", "changed_count",
+    }
+    assert expected_keys <= set(result.keys())
+    assert result["id_column"] == "Asset ID"
+
+
+# ─────────────────────────────────────────────────────────────────
+# manifest management
+# ─────────────────────────────────────────────────────────────────
+
+
+def test_load_manifest_empty_creates_default(tmp_path):
+    """load_manifest returns default structure when no file exists."""
+    with patch("cantaloupe.diff.MANIFEST_FILE", tmp_path / "manifest.json"):
+        with patch("cantaloupe.diff.MANIFEST_DIR", tmp_path):
+            manifest = load_manifest()
+            assert manifest == {"exports": []}
+
+
+def test_save_and_load_manifest(tmp_path):
+    """Round-trip: save a manifest then load it back."""
+    manifest_file = tmp_path / "manifest.json"
+    manifest_dir = tmp_path
+    with patch("cantaloupe.diff.MANIFEST_FILE", manifest_file):
+        with patch("cantaloupe.diff.MANIFEST_DIR", manifest_dir):
+            manifest = {"exports": [{"path": "/tmp/test.xlsx"}]}
+            save_manifest(manifest)
+
+            loaded = load_manifest()
+            assert loaded == manifest
+
+
+def test_record_export_appends(tmp_path):
+    """record_export appends to the exports list."""
+    manifest_file = tmp_path / "manifest.json"
+    with patch("cantaloupe.diff.MANIFEST_FILE", manifest_file):
+        with patch("cantaloupe.diff.MANIFEST_DIR", tmp_path):
+            record_export(Path("/tmp/a.xlsx"), 10, 1024)
+            record_export(Path("/tmp/b.xlsx"), 20, 2048)
+
+            manifest = load_manifest()
+            assert len(manifest["exports"]) == 2
+            assert manifest["exports"][0]["filename"] == "a.xlsx"
+            assert manifest["exports"][0]["row_count"] == 10
+            assert manifest["exports"][0]["file_size"] == 1024
+            assert manifest["exports"][1]["filename"] == "b.xlsx"
+            assert manifest["exports"][1]["row_count"] == 20
+
+
+def test_get_previous_export_returns_last(tmp_path):
+    """get_previous_export returns the path of the most recent export."""
+    manifest_file = tmp_path / "manifest.json"
+    a_path = tmp_path / "a.xlsx"
+    b_path = tmp_path / "b.xlsx"
+    a_path.write_text("dummy")
+    b_path.write_text("dummy")
+
+    with patch("cantaloupe.diff.MANIFEST_FILE", manifest_file):
+        with patch("cantaloupe.diff.MANIFEST_DIR", tmp_path):
+            record_export(a_path, 10, 100)
+            record_export(b_path, 20, 200)
+
+            prev = get_previous_export()
+            assert prev == b_path
+
+
+def test_get_previous_export_empty_manifest(tmp_path):
+    """Returns None when manifest has no exports."""
+    with patch("cantaloupe.diff.MANIFEST_FILE", tmp_path / "manifest.json"):
+        with patch("cantaloupe.diff.MANIFEST_DIR", tmp_path):
+            assert get_previous_export() is None
+
+
+def test_get_previous_export_missing_file(tmp_path):
+    """Returns None when the last export file no longer exists."""
+    manifest_file = tmp_path / "manifest.json"
+    ghost = tmp_path / "ghost.xlsx"
+
+    with patch("cantaloupe.diff.MANIFEST_FILE", manifest_file):
+        with patch("cantaloupe.diff.MANIFEST_DIR", tmp_path):
+            # Write manifest directly with a path that doesn't exist
+            save_manifest({"exports": [{"path": str(ghost), "timestamp": "2026-01-01"}]})
+            assert get_previous_export() is None
+
+
+def test_get_latest_exports_two(tmp_path):
+    """get_latest_exports returns two most recent existing files."""
+    manifest_file = tmp_path / "manifest.json"
+    a_path = tmp_path / "a.xlsx"
+    b_path = tmp_path / "b.xlsx"
+    c_path = tmp_path / "c.xlsx"
+    a_path.write_text("dummy")
+    b_path.write_text("dummy")
+    c_path.write_text("dummy")
+
+    with patch("cantaloupe.diff.MANIFEST_FILE", manifest_file):
+        with patch("cantaloupe.diff.MANIFEST_DIR", tmp_path):
+            record_export(a_path, 10, 100)
+            record_export(b_path, 20, 200)
+            record_export(c_path, 30, 300)
+
+            latest = get_latest_exports(2)
+            assert len(latest) == 2
+            assert latest[0] == c_path  # newest
+            assert latest[1] == b_path  # second newest
+
+
+def test_get_latest_exports_skips_missing(tmp_path):
+    """get_latest_exports skips entries whose file doesn't exist."""
+    manifest_file = tmp_path / "manifest.json"
+    a_path = tmp_path / "a.xlsx"
+    a_path.write_text("dummy")
+    ghost = tmp_path / "ghost.xlsx"
+
+    with patch("cantaloupe.diff.MANIFEST_FILE", manifest_file):
+        with patch("cantaloupe.diff.MANIFEST_DIR", tmp_path):
+            record_export(ghost, 10, 100)  # doesn't exist on disk
+            record_export(a_path, 20, 200)
+
+            latest = get_latest_exports(3)
+            assert len(latest) == 1
+            assert latest[0] == a_path
+
+
+def test_load_manifest_corrupt_json(tmp_path):
+    """Corrupt manifest JSON returns default empty structure."""
+    manifest_file = tmp_path / "manifest.json"
+    manifest_file.write_text("not valid json{{{")
+
+    with patch("cantaloupe.diff.MANIFEST_FILE", manifest_file):
+        with patch("cantaloupe.diff.MANIFEST_DIR", tmp_path):
+            manifest = load_manifest()
+            assert manifest == {"exports": []}
+
+
+# ─────────────────────────────────────────────────────────────────
+# CLI: diff command
+# ─────────────────────────────────────────────────────────────────
+
+
+@pytest.fixture
+def runner():
+    return CliRunner()
+
+
+def test_diff_command_basic(runner, tmp_path):
+    """diff command with explicit --from and --to files."""
+    old = _make_xlsx(tmp_path / "old.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "Machine 1"]],
+    )
+    new = _make_xlsx(tmp_path / "new.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "Machine 1"], ["A002", "Machine 2"]],
+    )
+
+    result = runner.invoke(cli, [
+        "diff",
+        "--from", str(old),
+        "--to", str(new),
+    ])
+    assert result.exit_code == 0
+    assert "Diff Report" in result.output
+    assert "1 machine(s) added" in result.output
+    assert "A002" in result.output
+
+
+def test_diff_command_no_changes(runner, tmp_path):
+    """diff command reports no changes for identical files."""
+    xlsx = _make_xlsx(tmp_path / "data.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "Machine 1"]],
+    )
+
+    result = runner.invoke(cli, [
+        "diff",
+        "--from", str(xlsx),
+        "--to", str(xlsx),
+    ])
+    assert result.exit_code == 0
+    assert "No changes" in result.output
+
+
+def test_diff_command_json_output(runner, tmp_path):
+    """--json flag outputs machine-readable JSON."""
+    old = _make_xlsx(tmp_path / "old.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "Machine 1"]],
+    )
+    new = _make_xlsx(tmp_path / "new.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "Machine 1"], ["A002", "Machine 2"]],
+    )
+
+    result = runner.invoke(cli, [
+        "diff",
+        "--from", str(old),
+        "--to", str(new),
+        "--json",
+    ])
+    assert result.exit_code == 0
+    data = json.loads(result.output)
+    assert data["added_count"] == 1
+    assert data["id_column"] == "Asset ID"
+
+
+def test_diff_command_missing_file(runner, tmp_path):
+    """Exits with error when --from file doesn't exist."""
+    new = _make_xlsx(tmp_path / "new.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "M1"]],
+    )
+
+    result = runner.invoke(cli, [
+        "diff",
+        "--from", str(tmp_path / "nonexistent.xlsx"),
+        "--to", str(new),
+    ])
+    assert result.exit_code == 1
+    assert "File not found" in result.output
+
+
+def test_diff_command_only_one_file_specified(runner, tmp_path):
+    """Specifying only --from (without --to) raises UsageError."""
+    old = _make_xlsx(tmp_path / "old.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "M1"]],
+    )
+
+    result = runner.invoke(cli, [
+        "diff",
+        "--from", str(old),
+    ])
+    assert result.exit_code == 2  # Click exits with 2 for UsageError
+    assert "Specify both" in result.output
+
+
+def test_diff_command_uses_manifest(runner, tmp_path):
+    """Without --from/--to, uses the two most recent exports."""
+    manifest_file = tmp_path / "manifest.json"
+    old = _make_xlsx(tmp_path / "old.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "Machine 1"]],
+    )
+    new = _make_xlsx(tmp_path / "new.xlsx",
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "Machine 1"], ["A002", "Machine 2"]],
+    )
+
+    with patch("cantaloupe.diff.MANIFEST_FILE", manifest_file):
+        with patch("cantaloupe.diff.MANIFEST_DIR", tmp_path):
+            record_export(old, 1, 100)
+            record_export(new, 2, 200)
+
+            result = runner.invoke(cli, ["diff"])
+            assert result.exit_code == 0
+            assert "Diff Report" in result.output
+            assert "1 machine(s) added" in result.output
+
+
+def test_diff_command_manifest_not_enough(runner, tmp_path):
+    """Exits with error when manifest has fewer than 2 exports."""
+    manifest_file = tmp_path / "manifest.json"
+
+    with patch("cantaloupe.diff.MANIFEST_FILE", manifest_file):
+        with patch("cantaloupe.diff.MANIFEST_DIR", tmp_path):
+            result = runner.invoke(cli, ["diff"])
+            assert result.exit_code == 1
+            assert "Need at least two exports" in result.output
+
+
+def test_diff_command_with_id_column(runner, tmp_path):
+    """--id-column specifies a custom key column."""
+    old = _make_xlsx(tmp_path / "old.xlsx",
+        headers=["Machine Number", "Name"],
+        rows=[["M001", "Machine 1"]],
+    )
+    new = _make_xlsx(tmp_path / "new.xlsx",
+        headers=["Machine Number", "Name"],
+        rows=[["M001", "Machine 1"], ["M002", "Machine 2"]],
+    )
+
+    result = runner.invoke(cli, [
+        "diff",
+        "--from", str(old),
+        "--to", str(new),
+        "--id-column", "Machine Number",
+    ])
+    assert result.exit_code == 0
+    assert "Machine Number" in result.output
+
+
+# ─────────────────────────────────────────────────────────────────
+# CLI: export --diff flag
+# ─────────────────────────────────────────────────────────────────
+
+
+def test_export_diff_flag_no_previous(runner, tmp_path):
+    """--diff flag with no previous export shows a warning."""
+    manifest_file = tmp_path / "manifest.json"
+
+    with patch("cantaloupe.diff.MANIFEST_FILE", manifest_file):
+        with patch("cantaloupe.diff.MANIFEST_DIR", tmp_path):
+            with patch.dict(
+                os.environ,
+                {"CANTALOUPE_EMAIL": "u@t.com", "CANTALOUPE_PASSWORD": "p"},
+            ):
+                with patch("src.download.download") as mock_download:
+                    mock_path = tmp_path / "export.xlsx"
+                    _make_xlsx(mock_path,
+                        headers=["Asset ID", "Name"],
+                        rows=[["A001", "M1"]],
+                    )
+                    mock_download.return_value = mock_path
+
+                    result = runner.invoke(cli, [
+                        "export", "--diff", "-o", str(tmp_path),
+                    ])
+                    assert result.exit_code == 0
+                    assert "Export complete" in result.output
+                    assert "No previous export" in result.output
+
+
+def test_export_diff_flag_with_previous(runner, tmp_path):
+    """--diff flag with a previous export shows the diff report."""
+    manifest_file = tmp_path / "manifest.json"
+    prev_path = tmp_path / "previous.xlsx"
+    new_path = tmp_path / "new_export.xlsx"
+
+    _make_xlsx(prev_path,
+        headers=["Asset ID", "Name"],
+        rows=[["A001", "Machine 1"]],
+    )
+
+    with patch("cantaloupe.diff.MANIFEST_FILE", manifest_file):
+        with patch("cantaloupe.diff.MANIFEST_DIR", tmp_path):
+            # Record a previous export
+            record_export(prev_path, 1, 100)
+
+            # Now the actual export would use a different timestamped name
+            # which we mock — but get_previous_export returns prev_path
+            _make_xlsx(new_path,
+                headers=["Asset ID", "Name"],
+                rows=[["A001", "Machine 1"], ["A002", "Machine 2"]],
+            )
+
+            with patch.dict(
+                os.environ,
+                {"CANTALOUPE_EMAIL": "u@t.com", "CANTALOUPE_PASSWORD": "p"},
+            ):
+                with patch("src.download.download") as mock_download:
+                    mock_download.return_value = new_path
+
+                    result = runner.invoke(cli, [
+                        "export", "--diff", "-o", str(tmp_path),
+                    ])
+                    assert result.exit_code == 0
+                    assert "Export complete" in result.output
+                    assert "Diff Report" in result.output
+                    assert "1 machine(s) added" in result.output
+
+
+def test_export_records_manifest(runner, tmp_path):
+    """Every export records to the manifest even without --diff."""
+    manifest_file = tmp_path / "manifest.json"
+
+    with patch("cantaloupe.diff.MANIFEST_FILE", manifest_file):
+        with patch("cantaloupe.diff.MANIFEST_DIR", tmp_path):
+            mock_path = tmp_path / "export.xlsx"
+            _make_xlsx(mock_path,
+                headers=["Asset ID", "Name"],
+                rows=[["A001", "M1"]],
+            )
+
+            with patch.dict(
+                os.environ,
+                {"CANTALOUPE_EMAIL": "u@t.com", "CANTALOUPE_PASSWORD": "p"},
+            ):
+                with patch("src.download.download") as mock_download:
+                    mock_download.return_value = mock_path
+
+                    result = runner.invoke(cli, [
+                        "export", "-o", str(tmp_path),
+                    ])
+                    assert result.exit_code == 0
+
+            # Verify manifest was updated
+            manifest = load_manifest()
+            assert len(manifest["exports"]) == 1
+            assert manifest["exports"][0]["filename"] == "export.xlsx"
+
+
+# ─────────────────────────────────────────────────────────────────
+# CLI: --help shows diff command
+# ─────────────────────────────────────────────────────────────────
+
+
+def test_cli_help_includes_diff(runner):
+    """--help now lists the diff command."""
+    result = runner.invoke(cli, ["--help"])
+    assert result.exit_code == 0
+    assert "diff" in result.output
+    # The help line shows the full docstring first line
+    assert "Compare two machine list" in result.output
+
+
+def test_export_help_includes_diff_flag(runner):
+    """export --help shows the --diff option."""
+    result = runner.invoke(cli, ["export", "--help"])
+    assert result.exit_code == 0
+    assert "--diff" in result.output
+
+
+def test_diff_help_shows_options(runner):
+    """diff --help shows all options."""
+    result = runner.invoke(cli, ["diff", "--help"])
+    assert result.exit_code == 0
+    assert "--from" in result.output
+    assert "--to" in result.output
+    assert "--id-column" in result.output
+    assert "--json" in result.output