feat(geometry): viewer-space conversion for citation highlighting

[hallelx2]

Makes the citation-highlight feature buildable: viewer-space coordinates so the frontend can draw an overlay on the rendered page, not just say "page 58".

Stacked on #10 (which is stacked on #9). Merge order: #9 → #10 → #11.

The good news first

The geometry was already correct. Verified rather than assumed: every coordinate the package reports — Char, Word, Line, Rect, Table.BBox, Table.CellsBBox — is already normalised. page.go translates the MediaBox origin to (0,0) and applies the /Rotate matrices, so the space matches Page.Width() x Page.Height() exactly. Rotated pages and non-zero-origin MediaBoxes are already handled, and no per-page fixups are needed.

Proved end-to-end: rendered 3M 2018 10-K page 58 at 150 DPI, projected real CellsBBox values onto the image, and the highlights land exactly on the intended rows.

What was actually missing

The contract was undocumented and untested. Nothing stopped a future change to coordinate handling from silently breaking an external consumer — the frontend would just start drawing boxes in the wrong place, with no test going red.

And there is one conversion every consumer must make and can easily invert: PDF user space is origin bottom-left, Y up; every web target (CSS, canvas, SVG, PDF.js) is origin top-left, Y down. Getting it backwards mirrors the overlay vertically — a failure that still looks plausible, because the boxes land on real rows, just the wrong ones. Better to do it once, here, with a test.

API

type ViewRect struct { Left, Top, Width, Height float64 }

func (b BBox) Viewport(pageHeight, scale float64) ViewRect
func (b BBox) Normalized(pageWidth, pageHeight float64) ViewRect

ViewRect is a distinct type from BBox on purpose — the two coordinate systems should not be mixable at a call site.

Viewport for a page rasterised at known DPI (scale = dpi/72) or a PDF.js getViewport({scale}). Normalized returns fractions of the page, which is what a resizable viewer wants: percentages stay correct at any zoom or container width, so the overlay survives a re-render without recomputation.

Tests

• Y flip pinned in both directions (a box near the page top must yield a near-zero Top).
• Scale factor, using the real coordinates of the "Less: Accumulated depreciation" row.
• The two APIs agree at several scales, so they cannot drift apart.
• Degenerate page returns the zero ViewRect rather than emitting NaN into a JSON payload.
• End-to-end pass over every word in a real fixture: every rect lands inside the page, right way up, non-degenerate.

go build, go vet, go test ./... -count=1 -race all green.

Frontend usage

r := table.CellsBBox[row][col].Normalized(page.Width(), page.Height())
// JSON {left, top, width, height} as fractions of the page;
// multiply by the rendered canvas size at paint time.

Granularity available today: whole table (Table.BBox), cell (Table.CellsBBox[i][j]), word (Page.Words), glyph (Page.Chars).

Follow-up

A citation usually spans several words across one or more lines. A helper that merges a run of word bboxes into the minimal set of line rectangles — rather than one union box that swallows the whole paragraph — would give a tighter highlight. Not needed for v1; noted on HAL-547.

Closes HAL-547

Summary by Sourcery

Introduce viewer-space rectangle support to enable accurate citation highlighting overlays based on PDF geometry.

New Features:

• Add ViewRect type to represent rectangles in viewer coordinates for frontend overlays.
• Provide BBox.Viewport helper to convert PDF user-space rectangles into scaled viewer coordinates for rendered pages.
• Provide BBox.Normalized helper to convert PDF user-space rectangles into resolution-independent viewer coordinate fractions.

Tests:

• Add unit tests verifying Y-axis flipping behavior and scaling correctness of Viewport.
• Add tests ensuring Normalized produces resolution-independent coordinates and handles degenerate page dimensions safely.
• Add end-to-end tests on real document fixtures to confirm converted rectangles are within page bounds and non-degenerate.

[sourcery-ai]

Reviewer's Guide

Adds a viewer-space rectangle type and conversion helpers so citation highlight coordinates can be consumed safely by web frontends, and backs the geometry contract with focused unit and end-to-end tests.

File-Level Changes

Change	Details	Files
Introduce a viewer-coordinate rectangle type and conversion methods from PDF user-space bounding boxes, including a normalized, resolution-independent form.	Define a ViewRect struct representing top-left–origin, Y-down rectangles via left/top/width/height. Add BBox.Viewport(pageHeight, scale) to convert PDF user-space boxes into scaled viewer coordinates with a Y-axis flip. Add BBox.Normalized(pageWidth, pageHeight) to express viewer rectangles as 0..1 fractions of page dimensions, guarding against non-positive page sizes.	geometry.go
Add unit tests that pin the coordinate conversion contract and verify correctness against real PDF fixtures.	Add tests for Y-axis flipping behavior at page top and bottom and for viewport scaling using known real-world coordinates. Verify Normalized output is resolution-independent and consistent with Viewport across multiple scales, and that degenerate pages return a zero ViewRect instead of NaN. Add an end-to-end test over words in a real PDF fixture to ensure all converted rectangles are inside the page, non-degenerate, and normalized to 0..1.	viewport_test.go

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📥 Commits

Reviewing files that changed from the base of the PR and between c69feb3 and f181bd6.

📒 Files selected for processing (2)

• geometry.go
• viewport_test.go

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[sourcery-ai]

Hey - I've left some high level feedback:

• Consider guarding Viewport and Normalized against obviously invalid inputs (e.g., negative page dimensions or scale) to fail fast instead of silently producing inverted or nonsensical viewer rectangles.
• You might want to offer convenience helpers that take a Page (or its width/height) directly for ViewRect conversion to reduce repetitive parameter passing and minimize call‑site mistakes with mismatched dimensions.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider guarding Viewport and Normalized against obviously invalid inputs (e.g., negative page dimensions or scale) to fail fast instead of silently producing inverted or nonsensical viewer rectangles.
- You might want to offer convenience helpers that take a Page (or its width/height) directly for ViewRect conversion to reduce repetitive parameter passing and minimize call‑site mistakes with mismatched dimensions.

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