# Task A4 Report: Build a Patched Binary in Isolation

## Status: COMPLETE

## TDD Evidence

### RED Phase
Appended the three A4 tests to `tests/testthat/test-patches.R` and ran:

```
[ FAIL 3 | WARN 0 | SKIP 0 | PASS 26 ]
```

Failures were exactly as expected:
- `could not find function "configure_args_to_build_args"`
- `could not find function "apply_source_patch"`
- `Can't find binding for 'download_cran_source'` (local_mocked_bindings requires the function to exist in the package namespace)

### GREEN Phase
Appended four helpers to `R/patches.R`:
- `download_cran_source` — fetches source tarball from CRAN current or Archive URL
- `apply_source_patch` — runs `patch -p1 --forward --batch`, returns logical
- `configure_args_to_build_args` — formats configure args for `pkgbuild::build(args=...)`
- `build_patched_binary` — orchestrates download → untar → patch → env-scoped build

All 30 tests pass:

```
[ FAIL 0 | WARN 0 | SKIP 0 | PASS 30 ]
```

## Commit

SHA: `944c7ec`
Subject: `feat(patches): build a patched binary in isolation from CRAN source`
Branch: `feat/package-patching`
Repo: `/Users/pjs/git/codefloe.com/rpkgs/bincraftr`

## Files Modified

- `R/patches.R` — 4 new helpers appended after `describe_patch`
- `tests/testthat/test-patches.R` — 3 new `test_that` blocks appended

## Notes / Concerns

- All new helpers carry `@keywords internal` exactly as specified.
- The `apply_source_patch` test uses a patch targeting `missing.txt` which does not exist in the temp dir, so `patch` exits non-zero → `FALSE`. This correctly validates the failure path without any compilation.
- The `build_patched_binary` test mocks `download_cran_source` to return `NULL`, so no network or compiler is involved — safe on macOS CI.
- Implementation is verbatim from the brief; no deviations.
- `--no-verify` was used as instructed (broken pre-commit hook in this environment).