ewah: factor out bitmap growth

We auto-grow bitmaps when somebody asks to set a bit whose position is
outside of our currently allocated range. Other operations besides
single bit-setting might need to do this, too, so let's pull it into its
own function.

Note that we change the semantics a little: you now ask for the number
of words you'd like to have, not the id of the block you'd like to write
to.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

ewah/bitmap.c

@@ -35,18 +35,22 @@ struct bitmap *bitmap_new(void)
 	return bitmap_word_alloc(32);
 }
 
-void bitmap_set(struct bitmap *self, size_t pos)
+static void bitmap_grow(struct bitmap *self, size_t word_alloc)
 {
-	size_t block = EWAH_BLOCK(pos);
-
-	if (block >= self->word_alloc) {
+	if (word_alloc > self->word_alloc) {
 		size_t old_size = self->word_alloc;
-		self->word_alloc = block ? block * 2 : 1;
+		self->word_alloc = word_alloc * 2;
 		REALLOC_ARRAY(self->words, self->word_alloc);
 		memset(self->words + old_size, 0x0,
 			(self->word_alloc - old_size) * sizeof(eword_t));
 	}
+}
 
+void bitmap_set(struct bitmap *self, size_t pos)
+{
+	size_t block = EWAH_BLOCK(pos);
+
+	bitmap_grow(self, block + 1);
 	self->words[block] |= EWAH_MASK(pos);
 }