Module hunk 

MG.18b: locating the hunk at a cursor, and turning it into a patch.

Pure functions over diff text. No buffer, no store, no Editor — magit.md §7.2’s substrate-helper pattern, and what lets MG.22 relocate this into magit-hunk-mode rather than rewrite it.

Why lines arrive through an accessor

hunk_at_with pulls lines one at a time by index rather than taking a slice. Its production caller reads a buffer snapshot, and a *magit:diff* buffer can hold tens of thousands of lines — materialising all of them to stage one hunk would put an O(document) copy on a keystroke path (paramount #1). Through the accessor the read is O(distance to the file header + hunk body), which is the work the answer actually costs. The slice form below is the same parser, adapted for tests.

Why the buffer text is the source of truth

magit.md §7.5: hunk boundaries are already derived from buffer text. ]c / [c walk hunk_lines in magit_core_mode.rs — a raw scan for @@ / diff --git, identical in every magit buffer, consulting no cache. Staging reads the same text, so navigation and staging cannot disagree about where a hunk begins. A parsed-hunk cache (§7.2 records one as deliberately absent) would give ]c one boundary set and s another with nothing forcing agreement.

Why headers are copied, never reconstructed

HunkPatch::header holds the file’s header lines verbatim — diff --git, index, mode changes, ---, +++. Rebuilding them from a parsed path would have to re-derive git’s own quoting for paths with spaces or non-ASCII bytes, and would silently drop rename and mode metadata. Copying cannot get any of that wrong.

Structs

HunkPatch

One hunk, plus the file header needed to apply it on its own.

SourcePos

A position in the SOURCE FILE, resolved from a cursor sitting on a diff row. 0-based row plus a 0-based BYTE offset within it, matching lattice_protocol::position::Position’s own { line, byte } shape.

Enums

ApplyDirection

Which way a patch will be handed to git apply, and therefore which side of it the target already matches.

Functions

hunk_at_with

hunk_ordinal_at

MG.18d: the 0-based index of the hunk whose header sits at header_row, among the hunks of the file it belongs to.

path_at_cursor

Locate the hunk containing cursor, reading lines through read.

source_position_at

MG.50: the SOURCE position the cursor is looking at, inside a diff.