Diffs

S19 ships the diff renderer wired into the S18 single-commit page. The same renderer will host S20's compare view and S22's PR files-changed once those sprints land.

Pipeline

git diff (--patch)        →  []byte
internal/repos/diff/source     produces the raw patch bytes
                                  ▼
internal/repos/diff/parse       wraps go-gitdiff into our types
                                  ▼
internal/repos/diff/render      emits HTML for the templates

The shape is library-on-the-inside, package-on-the-outside: the public surface exposes our Diff/File/Hunk/Line structs so we can swap the parser later without breaking callers.

Source helpers (source/)

Options carries IgnoreWhitespace (-w) and FindRenames (-M -C). Whitespace toggles re-source rather than parser-side filter — git's -w respects language quirks (Python indentation, etc.) more correctly than post-processing hunks.

The --root flag on FromCommit is what lets the initial parentless commit emit its files (against the empty tree); without it the parser sees nothing.

Parser (parse/)

Wraps github.com/bluekeyes/go-gitdiff/gitdiff. Translates each *gitdiff.File into our File, expanding TextFragments into Hunks whose Lines carry one of four LineKinds:

Old/new line numbers are populated based on kind:

• Context: both populated
• Add: only NewLineNo
• Delete: only OldLineNo

The renderer reads SizeBytes (sum of line content) for the "per-file too large" decision — cheap to compute during parse.

Renderer (render/)

Two modes share the same RenderFile skeleton (header → hunks → lines):

• Unified — old/new line numbers in two adjacent <td> cells, content in the third. The +/-/space marker is prepended to the content for visual consistency with git diff.
• Split — left side (old) and right side (new) in separate columns. Adds and deletes pair row-by-row; trailing additions on either side leave the other column padded.

Per-file collapse + whole-diff truncate

The collapsed <details> currently renders the hunks inside the toggle (lazy at the user's click but still inline). The spec calls for a separate /diff-fragment endpoint that fetches per-file hunks on demand; that's a polish for S36 when we have a real big-PR workload to bench against.

Binary + image

Binary files emit a shithub-diff-binary placeholder. Image files (by extension) render an shithub-diff-image placeholder note — the side-by-side preview wires once the renderer accepts ref+path context for /raw/... URLs (deferred polish; the commit page already links the changed file in the file table).

Highlighting

The renderer currently HTML-escapes line content; the file-level <a> link to the blob view (which uses the S17 Chroma highlighter) is the path users follow when they want syntax highlighting on a specific change. Per-line Chroma highlighting in the diff itself is deferred — escape is the floor; richer rendering is polish.

Routes

The S19 changes are entirely additive to the S18 commit page. New query parameters:

The toggles emit links that preserve the other parameter so users don't lose a mode when toggling whitespace and vice versa.

Tests

• parse/parse_test.go — happy path, rename, binary, empty, multi-file.
• render/render_test.go — unified + split classes present, binary placeholder, rename header, too-large collapse, empty-diff label.

Pitfalls handled

• Initial-commit diff emits nothing without --root — the source helper passes the flag.
• go-gitdiff returns (nil, _, io.EOF) for empty input — treated as "no files," no error.
• HTML escape on every line — content goes through html.EscapeString; only the renderer's wrapper markup is raw.
• Whitespace toggle cache key — currently no cache; when one lands in S36, the cache key must include IgnoreWhitespace.
• Trailing-newline preservation — parse.trimNewline strips one \n; multi-newline trailing data is preserved verbatim.

Deferred polish (tracked)

• POST /{owner}/{repo}/diff-fragment?... endpoint — per-file lazy fetch for whole-diff-truncated cases. Track in S36 with the rest of the diff/blame caching work; current <details> inline expand is the floor.
• Per-line Chroma highlighting in diff hunks — every line currently HTML-escapes; richer rendering is post-MVP. The Chroma helper (internal/repos/highlight) is the same one the blob view uses, so the wiring is mechanical when desired.
• Image diff side-by-side preview — needs /raw/<sha>/<path> threading into the renderer. Current placeholder is honest; the file table already links the new path's blob.
• Diff cache keyed on (base_sha, head_sha) — S36, with the rest of the immutable-content cache layer.