From 8d25663d704d1216d2fd5db5fd3aa431b8c58268 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ren=C3=A9=20Scharfe?= <l.s.r@web.de>
Date: Sun, 25 Feb 2024 12:39:44 +0100
Subject: [PATCH] mem-pool: add mem_pool_strfmt()
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Add a function for building a string, printf style, using a memory pool.
It uses the free space in the current block in the first attempt.  If
that suffices then the result can already be used without copying or
reformatting.

For strings that are significantly shorter on average than the block
size (ca. 1 MiB by default) this is the case most of the time, leading
to a better perfomance than a solution that doesn't access mem-pool
internals.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 mem-pool.c | 39 +++++++++++++++++++++++++++++++++++++++
 mem-pool.h |  5 +++++
 2 files changed, 44 insertions(+)

diff --git a/mem-pool.c b/mem-pool.c
index c7d6256020..2078c22b09 100644
--- a/mem-pool.c
+++ b/mem-pool.c
@@ -107,6 +107,45 @@ void *mem_pool_alloc(struct mem_pool *pool, size_t len)
 	return r;
 }
 
+static char *mem_pool_strvfmt(struct mem_pool *pool, const char *fmt,
+			      va_list ap)
+{
+	struct mp_block *block = pool->mp_block;
+	char *next_free = block ? block->next_free : NULL;
+	size_t available = block ? block->end - block->next_free : 0;
+	va_list cp;
+	int len, len2;
+	char *ret;
+
+	va_copy(cp, ap);
+	len = vsnprintf(next_free, available, fmt, cp);
+	va_end(cp);
+	if (len < 0)
+		BUG("your vsnprintf is broken (returned %d)", len);
+
+	ret = mem_pool_alloc(pool, len + 1);  /* 1 for NUL */
+
+	/* Shortcut; relies on mem_pool_alloc() not touching buffer contents. */
+	if (ret == next_free)
+		return ret;
+
+	len2 = vsnprintf(ret, len + 1, fmt, ap);
+	if (len2 != len)
+		BUG("your vsnprintf is broken (returns inconsistent lengths)");
+	return ret;
+}
+
+char *mem_pool_strfmt(struct mem_pool *pool, const char *fmt, ...)
+{
+	va_list ap;
+	char *ret;
+
+	va_start(ap, fmt);
+	ret = mem_pool_strvfmt(pool, fmt, ap);
+	va_end(ap);
+	return ret;
+}
+
 void *mem_pool_calloc(struct mem_pool *pool, size_t count, size_t size)
 {
 	size_t len = st_mult(count, size);
diff --git a/mem-pool.h b/mem-pool.h
index fe7507f022..d1c66413ec 100644
--- a/mem-pool.h
+++ b/mem-pool.h
@@ -47,6 +47,11 @@ void *mem_pool_calloc(struct mem_pool *pool, size_t count, size_t size);
 char *mem_pool_strdup(struct mem_pool *pool, const char *str);
 char *mem_pool_strndup(struct mem_pool *pool, const char *str, size_t len);
 
+/*
+ * Allocate memory from the memory pool and format a string into it.
+ */
+char *mem_pool_strfmt(struct mem_pool *pool, const char *fmt, ...);
+
 /*
  * Move the memory associated with the 'src' pool to the 'dst' pool. The 'src'
  * pool will be empty and not contain any memory. It still needs to be free'd