The rules are basically:
- each bitmap is a series of counts of runs of 0/1
- each count is one of our standard varints
- each bitmap must have at least one initial count of
zeroes (which may itself be a zero-length count, if the
first bit is set)
- a zero-length count anywhere else marks the end of
the bitmap
For a sparse bitmap, these will tend to be quite short,
because long runs are encoded as fairly small counts. The
worst case is an alternate 0/1/0/1 bitmap, where we will
spend a full byte to specify each bit (thus bloating it by a
factor of 8 over an uncompressed bitmap).
Signed-off-by: Jeff King <peff@xxxxxxxx>
---
t/helper/test-tree-bitmap.c | 105 +++++++++++++++++++++++++++++++-----
1 file changed, 91 insertions(+), 14 deletions(-)
diff --git a/t/helper/test-tree-bitmap.c b/t/helper/test-tree-bitmap.c
index 6f8833344a..36f19ed464 100644
--- a/t/helper/test-tree-bitmap.c
+++ b/t/helper/test-tree-bitmap.c
@@ -3,6 +3,7 @@
#include "diffcore.h"
#include "argv-array.h"
#include "ewah/ewok.h"
+#include "varint.h"
/* map of pathnames to bit positions */
struct pathmap_entry {
@@ -123,6 +124,49 @@ static void collect_paths(struct hashmap *paths)
free(sorted);
}
+static void strbuf_add_varint(struct strbuf *out, uintmax_t val)
+{
+ size_t len;
+ strbuf_grow(out, 16); /* enough for any varint */
+ len = encode_varint(val, (unsigned char *)out->buf + out->len);
+ strbuf_setlen(out, out->len + len);
+}
+
+static void bitmap_to_rle(struct strbuf *out, struct bitmap *bitmap)
+{
+ int curval = 0; /* count zeroes, then ones, then zeroes, etc */
+ size_t run = 0;
+ size_t word;
+ size_t orig_len = out->len;
+
+ for (word = 0; word < bitmap->word_alloc; word++) {
+ int bit;
+
+ for (bit = 0; bit < BITS_IN_EWORD; bit++) {
+ int val = !!(bitmap->words[word] & (((eword_t)1) << bit));
+ if (val == curval)
+ run++;
+ else {
+ strbuf_add_varint(out, run);
+ curval = 1 - curval; /* flip 0/1 */
+ run = 1;
+ }
+ }
+ }
+
+ /*
+ * complete the run, but do not bother with trailing zeroes, unless we
+ * failed to write even an initial run of 0's.
+ */
+ if (curval && run)
+ strbuf_add_varint(out, run);
+ else if (orig_len == out->len)
+ strbuf_add_varint(out, 0);
+
+ /* signal end-of-input with an empty run */
+ strbuf_add_varint(out, 0);
+}
+
/* generate the bitmap for a single commit */
static void generate_bitmap(struct diff_queue_struct *q,
struct diff_options *opts,
@@ -130,7 +174,6 @@ static void generate_bitmap(struct diff_queue_struct *q,
{
struct walk_paths_data *data = vdata;
struct bitmap *bitmap = bitmap_new();
- struct ewah_bitmap *ewah;
struct strbuf out = STRBUF_INIT;
size_t i;
@@ -148,8 +191,7 @@ static void generate_bitmap(struct diff_queue_struct *q,
bitmap_set(bitmap, entry->pos);
}
- ewah = bitmap_to_ewah(bitmap);
- ewah_serialize_strbuf(ewah, &out);
+ bitmap_to_rle(&out, bitmap);
fwrite(data->commit->object.oid.hash, 1, GIT_SHA1_RAWSZ, stdout);
fwrite(out.buf, 1, out.len, stdout);
@@ -160,7 +202,6 @@ static void generate_bitmap(struct diff_queue_struct *q,
(unsigned)out.len);
strbuf_release(&out);
- ewah_free(ewah);
bitmap_free(bitmap);
}
@@ -181,6 +222,51 @@ static void show_path(size_t pos, void *data)
printf("%s\n", paths[pos]);
}
+static size_t rle_each_bit(const unsigned char *in, size_t len,
+ void (*fn)(size_t, void *), void *data)
+{
+ int curval = 0; /* look for zeroes first, then ones, etc */
+ const unsigned char *cur = in;
+ const unsigned char *end = in + len;
+ size_t pos;
+
+ /* we always have a first run, even if it's 0 zeroes */
+ pos = decode_varint(&cur);
+
+ /*
+ * ugh, varint does not seem to have a way to prevent reading past
+ * the end of the buffer. We'll do a length check after each one,
+ * so the worst case is bounded.
+ */
+ if (cur > end) {
+ error("input underflow in rle");
+ return len;
+ }
+
+ while (1) {
+ size_t run = decode_varint(&cur);
+
+ if (cur > end) {
+ error("input underflow in rle");
+ return len;
+ }
+
+ if (!run)
+ break; /* empty run signals end */
+
+ curval = 1 - curval; /* flip 0/1 */
+ if (curval) {
+ /* we have a run of 1's; deliver them */
+ size_t i;
+ for (i = 0; i < run; i++)
+ fn(pos + i, data);
+ }
+ pos += run;
+ }
+
+ return cur - in;
+}
+
static void do_dump(void)
{
struct strbuf in = STRBUF_INIT;
@@ -219,7 +305,6 @@ static void do_dump(void)
/* read the bitmap for each commit */
while (remain) {
struct object_id oid;
- struct ewah_bitmap *ewah;
ssize_t len;
if (remain < GIT_SHA1_RAWSZ) {
@@ -230,17 +315,9 @@ static void do_dump(void)
cur += GIT_SHA1_RAWSZ;
remain -= GIT_SHA1_RAWSZ;
- ewah = ewah_new();
- len = ewah_read_mmap(ewah, cur, remain);
- if (len < 0) {
- ewah_free(ewah);
- goto out;
- }
-
printf("%s\n", oid_to_hex(&oid));
- ewah_each_bit(ewah, show_path, paths);
+ len = rle_each_bit((const unsigned char *)cur, remain, show_path, paths);
- ewah_free(ewah);
cur += len;
remain -= len;
}
--
2.19.1.550.g7610f1eecb
