[PATCH 1/4] Add a (very) barebones Bloom filter implementation

["SZEDER Gábor" <szeder.dev@gmail.com>]

---
 Makefile       |   1 +
 bloom-filter.c | 103 +++++++++++++++++++++++++++++++++++++++++++++++++
 bloom-filter.h |  39 +++++++++++++++++++
 3 files changed, 143 insertions(+)
 create mode 100644 bloom-filter.c
 create mode 100644 bloom-filter.h

diff --git a/Makefile b/Makefile
index 13e1c52478..850eafb3ee 100644
--- a/Makefile
+++ b/Makefile
@@ -827,6 +827,7 @@ LIB_OBJS += base85.o
 LIB_OBJS += bisect.o
 LIB_OBJS += blame.o
 LIB_OBJS += blob.o
+LIB_OBJS += bloom-filter.o
 LIB_OBJS += branch.o
 LIB_OBJS += bulk-checkin.o
 LIB_OBJS += bundle.o
diff --git a/bloom-filter.c b/bloom-filter.c
new file mode 100644
index 0000000000..7dce0e35fa
--- /dev/null
+++ b/bloom-filter.c
@@ -0,0 +1,103 @@
+#include "cache.h"
+#include "bloom-filter.h"
+
+void bloom_filter_init(struct bloom_filter *bf, uint32_t bit_size)
+{
+	if (bit_size % CHAR_BIT)
+		BUG("invalid size for bloom filter");
+
+	bf->nr_entries = 0;
+	bf->bit_size = bit_size;
+	bf->bits = xmalloc(bit_size / CHAR_BIT);
+}
+
+void bloom_filter_free(struct bloom_filter *bf)
+{
+	bf->nr_entries = 0;
+	bf->bit_size = 0;
+	FREE_AND_NULL(bf->bits);
+}
+
+
+void bloom_filter_set_bits(struct bloom_filter *bf, const uint32_t *offsets,
+			   int nr_offsets, int nr_entries)
+{
+	int i;
+	for (i = 0; i < nr_offsets; i++) {
+		uint32_t byte_offset = (offsets[i] % bf->bit_size) / CHAR_BIT;
+		unsigned char mask = 1 << offsets[i] % CHAR_BIT;
+		bf->bits[byte_offset] |= mask;
+	}
+	bf->nr_entries += nr_entries;
+}
+
+int bloom_filter_check_bits(struct bloom_filter *bf, const uint32_t *offsets,
+			    int nr)
+{
+	int i;
+	for (i = 0; i < nr; i++) {
+		uint32_t byte_offset = (offsets[i] % bf->bit_size) / CHAR_BIT;
+		unsigned char mask = 1 << offsets[i] % CHAR_BIT;
+		if (!(bf->bits[byte_offset] & mask))
+			return 0;
+	}
+	return 1;
+}
+
+
+void bloom_filter_add_hash(struct bloom_filter *bf, const unsigned char *hash)
+{
+	uint32_t offsets[GIT_MAX_RAWSZ / sizeof(uint32_t)];
+	hashcpy((unsigned char*)offsets, hash);
+	bloom_filter_set_bits(bf, offsets,
+			     the_hash_algo->rawsz / sizeof(*offsets), 1);
+}
+
+int bloom_filter_check_hash(struct bloom_filter *bf, const unsigned char *hash)
+{
+	uint32_t offsets[GIT_MAX_RAWSZ / sizeof(uint32_t)];
+	hashcpy((unsigned char*)offsets, hash);
+	return bloom_filter_check_bits(bf, offsets,
+			the_hash_algo->rawsz / sizeof(*offsets));
+}
+
+void hashxor(const unsigned char *hash1, const unsigned char *hash2,
+	     unsigned char *out)
+{
+	int i;
+	for (i = 0; i < the_hash_algo->rawsz; i++)
+		out[i] = hash1[i] ^ hash2[i];
+}
+
+/* hardcoded for now... */
+static GIT_PATH_FUNC(git_path_bloom, "objects/info/bloom")
+
+int bloom_filter_load(struct bloom_filter *bf)
+{
+	int fd = open(git_path_bloom(), O_RDONLY);
+
+	if (fd < 0)
+		return -1;
+
+	read_in_full(fd, &bf->nr_entries, sizeof(bf->nr_entries));
+	read_in_full(fd, &bf->bit_size, sizeof(bf->bit_size));
+	if (bf->bit_size % CHAR_BIT)
+		BUG("invalid size for bloom filter");
+	bf->bits = xmalloc(bf->bit_size / CHAR_BIT);
+	read_in_full(fd, bf->bits, bf->bit_size / CHAR_BIT);
+
+	close(fd);
+
+	return 0;
+}
+
+void bloom_filter_write(struct bloom_filter *bf)
+{
+	int fd = xopen(git_path_bloom(), O_WRONLY | O_CREAT | O_TRUNC, 0666);
+
+	write_in_full(fd, &bf->nr_entries, sizeof(bf->nr_entries));
+	write_in_full(fd, &bf->bit_size, sizeof(bf->bit_size));
+	write_in_full(fd, bf->bits, bf->bit_size / CHAR_BIT);
+
+	close(fd);
+}
diff --git a/bloom-filter.h b/bloom-filter.h
new file mode 100644
index 0000000000..94d0af1708
--- /dev/null
+++ b/bloom-filter.h
@@ -0,0 +1,39 @@
+#ifndef BLOOM_FILTER_H
+#define BLOOM_FILTER_H
+
+#include "git-compat-util.h"
+
+struct bloom_filter {
+	uint32_t nr_entries;
+	uint32_t bit_size;
+	unsigned char *bits;
+};
+
+
+void bloom_filter_init(struct bloom_filter *bf, uint32_t bit_size);
+void bloom_filter_free(struct bloom_filter *bf);
+
+void bloom_filter_set_bits(struct bloom_filter *bf, const uint32_t *offsets,
+			   int nr_offsets, int nr_enries);
+int bloom_filter_check_bits(struct bloom_filter *bf, const uint32_t *offsets,
+			    int nr);
+
+/*
+ * Turns the given (SHA1) hash into 5 unsigned ints, and sets the bits at
+ * those positions (modulo the bitmap's size) in the Bloom filter.
+ */
+void bloom_filter_add_hash(struct bloom_filter *bf, const unsigned char *hash);
+/*
+ * Turns the given (SHA1) hash into 5 unsigned ints, and checks the bits at
+ * those positions (modulo the bitmap's size) in the Bloom filter.
+ * Returns 1 if all those bits are set, 0 otherwise.
+ */
+int bloom_filter_check_hash(struct bloom_filter *bf, const unsigned char *hash);
+
+void hashxor(const unsigned char *hash1, const unsigned char *hash2,
+	     unsigned char *out);
+
+int bloom_filter_load(struct bloom_filter *bf);
+void bloom_filter_write(struct bloom_filter *bf);
+
+#endif
-- 
2.19.1.409.g0a0ee5eb6b