Amend the lib/ code for easier inclusion in other programs

This introduces no functional changes, but allows the lib/ code to be
used as-is in other programs without patching these files directly,
instead those programs can define a few variables to inject their
custom code into this code.

With these changes the git project can use this code without any
modifications to the upstream code. The corresponding git mailing list
thread discussing that is at "[PATCH/RFC 0/3] Use
sha1collisiondetection as a submodule"[1]. The defines used by git in
that patch are:

    -DSHA1DC_NO_STANDARD_INCLUDES \
    -DSHA1DC_INIT_SAFE_HASH_DEFAULT=0 \
    -DSHA1DC_CUSTOM_INCLUDE_SHA1_C="\"cache.h\"" \
    -DSHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_C="\"../sha1dc_git.c\"" \
    -DSHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_H="\"../sha1dc_git.h\"" \
    -DSHA1DC_CUSTOM_INCLUDE_UBC_CHECK_C="\"git-compat-util.h\""

I.e. all of these except SHA1DC_CUSTOM_TRAILING_INCLUDE_UBC_CHECK_[CH]
are used by git. I've merely added those for completeness since other
projects doing similar customization might want to use them.

The monkeypatches the git project was using previously were:

  - https://github.com/git/git/commit/45a574eec8
  - https://github.com/git/git/commit/c0c20060af
  - https://github.com/git/git/commit/8325e43b82

The entire paragraph being added to the README file was authored by
Dan Shumow. See
https://github.com/cr-marcstevens/sha1collisiondetection/pull/31.

1. <20170517113824.31700-1-avarab@gmail.com>
   (https://public-inbox.org/git/20170517113824.31700-1-avarab@gmail.com/)

1 files changed, 15 insertions, 1 deletions

diff --git a/lib/sha1.c b/lib/sha1.c
index c5e45a9..26516b1 100644
--- a/lib/sha1.c
+++ b/lib/sha1.c
@@ -5,10 +5,20 @@
 * https://opensource.org/licenses/MIT
 ***/
 
+#ifndef SHA1DC_NO_STANDARD_INCLUDES
 #include <string.h>
 #include <memory.h>
 #include <stdio.h>
 #include <stdlib.h>
+#endif
+
+#ifdef SHA1DC_CUSTOM_INCLUDE_SHA1_C
+#include SHA1DC_CUSTOM_INCLUDE_SHA1_C
+#endif
+
+#ifndef SHA1DC_INIT_SAFE_HASH_DEFAULT
+#define SHA1DC_INIT_SAFE_HASH_DEFAULT 1
+#endif
 
 #include "sha1.h"
 #include "ubc_check.h"
@@ -1689,7 +1699,7 @@ void SHA1DCInit(SHA1_CTX* ctx)
 	ctx->ihv[3] = 0x10325476;
 	ctx->ihv[4] = 0xC3D2E1F0;
 	ctx->found_collision = 0;
-	ctx->safe_hash = 1;
+	ctx->safe_hash = SHA1DC_INIT_SAFE_HASH_DEFAULT;
 	ctx->ubc_check = 1;
 	ctx->detect_coll = 1;
 	ctx->reduced_round_coll = 0;
@@ -1824,3 +1834,7 @@ int SHA1DCFinal(unsigned char output[20], SHA1_CTX *ctx)
 	output[19] = (unsigned char)(ctx->ihv[4]);
 	return ctx->found_collision;
 }
+
+#ifdef SHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_C
+#include SHA1DC_CUSTOM_TRAILING_INCLUDE_SHA1_C
+#endif