[PATCH 4/6] pack-bitmap.c: factor out manual `map_pos` manipulation

[Taylor Blau <me@ttaylorr.com>]

The pack-bitmap internals use a combination of a `map` and `map_pos`
variable to keep a pointer to a memory mapped region of the `.bitmap`
file, as well as the position within that file, respectively.

Reads within the memory mapped region are meant to mimic file reads,
where each read adjusts the offset of the file descriptor accordingly.
This functionality is implemented by adjusting the `map_pos` variable
after each read.

Factor out a function similar to seek() which adjusts the `map_pos`
variable for us. Since the bitmap code only needs to adjust relative to
the beginning of the file as well as the current position, we do not
need to support any "whence" values outside of SEEK_SET and SEEK_CUR.

Extracting out this operation into a separate function allows us to add
some additional safety checks, such as ensuring that adding to `map_pos`
does not overflow `size_t`, and that the resulting `map_pos` value is
in bounds of the mapped region.

The subsequent commit will rewrite all manual manipulation of the
`map_pos` variable in terms of the new `bitmap_index_seek()`.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 pack-bitmap.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/pack-bitmap.c b/pack-bitmap.c
index 1d12f90ff9..fabcf01c14 100644
--- a/pack-bitmap.c
+++ b/pack-bitmap.c
@@ -134,6 +134,28 @@ static struct ewah_bitmap *lookup_stored_bitmap(struct stored_bitmap *st)
 	return composed;
 }
 
+static size_t bitmap_index_seek(struct bitmap_index *bitmap_git, size_t offset,
+				int whence)
+{
+	switch (whence) {
+	case SEEK_SET:
+		bitmap_git->map_pos = offset;
+		break;
+	case SEEK_CUR:
+		bitmap_git->map_pos = st_add(bitmap_git->map_pos, offset);
+		break;
+	default:
+		BUG("unhandled seek whence: %d", whence);
+	}
+
+	if (bitmap_git->map_pos >= bitmap_git->map_size)
+		BUG("bitmap position exceeds size (%"PRIuMAX" >= %"PRIuMAX")",
+		    (uintmax_t)bitmap_git->map_pos,
+		    (uintmax_t)bitmap_git->map_size);
+
+	return bitmap_git->map_pos;
+}
+
 /*
  * Read a bitmap from the current read position on the mmaped
  * index, and increase the read position accordingly
-- 
2.40.0.77.gd564125b3f