Subject: [PATCH v2 18/18] misc::varint: reimplement as test balloon for Rust

From: "Patrick Steinhardt via GitGitGadget" <[email-masked]>
Date: Wed, 17 Sep 2025 01:16:38 +0000

From: Patrick Steinhardt <[email-masked]>

Implement a trivial test balloon for our Rust build infrastructure by
reimplementing the "varint.c" subsystem in Rust. This subsystem is
chosen because it is trivial to convert and because it doesn't have any
dependencies to other components of Git.

If support for Rust is enabled, we stop compiling "varint.c" and instead
compile and use "src/varint.rs".

Signed-off-by: Patrick Steinhardt <[email-masked]>
Signed-off-by: Ezekiel Newren <[email-masked]>
---
 Makefile                |  2 +
 meson.build             |  7 +++-
 rust/misc/src/lib.rs    |  1 +
 rust/misc/src/varint.rs | 92 +++++++++++++++++++++++++++++++++++++++++
 4 files changed, 101 insertions(+), 1 deletion(-)
 create mode 100644 rust/misc/src/varint.rs

diff --git a/Makefile b/Makefile
index c363a6365d..9d50f88809 100644
--- a/Makefile
+++ b/Makefile
@@ -1259,7 +1259,9 @@ LIB_OBJS += urlmatch.o
 LIB_OBJS += usage.o
 LIB_OBJS += userdiff.o
 LIB_OBJS += utf8.o
+ifeq ($(WITH_RUST),false)
 LIB_OBJS += varint.o
+endif
 LIB_OBJS += version.o
 LIB_OBJS += versioncmp.o
 LIB_OBJS += walker.o
diff --git a/meson.build b/meson.build
index 6911625f3b..1b191b12a3 100644
--- a/meson.build
+++ b/meson.build
@@ -571,7 +571,6 @@ libgit_sources = [
   'usage.c',
   'userdiff.c',
   'utf8.c',
-  'varint.c',
   'version.c',
   'versioncmp.c',
   'walker.c',
@@ -591,6 +590,12 @@ libgit_sources = [
   'xdiff/xutils.c',
 ]
 
+if not with_rust
+  libgit_sources += [
+    'varint.c',
+  ]
+endif
+
 libgit_sources += custom_target(
   input: 'command-list.txt',
   output: 'command-list.h',
diff --git a/rust/misc/src/lib.rs b/rust/misc/src/lib.rs
index e69de29bb2..9da70d8b57 100644
--- a/rust/misc/src/lib.rs
+++ b/rust/misc/src/lib.rs
@@ -0,0 +1 @@
+pub mod varint;
diff --git a/rust/misc/src/varint.rs b/rust/misc/src/varint.rs
new file mode 100644
index 0000000000..10c83e1f43
--- /dev/null
+++ b/rust/misc/src/varint.rs
@@ -0,0 +1,92 @@
+#[no_mangle]
+pub unsafe extern "C" fn decode_varint(bufp: *mut *const u8) -> usize {
+    let mut buf = *bufp;
+    let mut c = *buf;
+    let mut val = usize::from(c & 127);
+
+    buf = buf.add(1);
+
+    while (c & 128) != 0 {
+        val = val.wrapping_add(1);
+        if val == 0 || val.leading_zeros() < 7 {
+            return 0; // overflow
+        }
+
+        c = *buf;
+        buf = buf.add(1);
+
+        val = (val << 7) + usize::from(c & 127);
+    }
+
+    *bufp = buf;
+    val
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn encode_varint(value: usize, buf: *mut u8) -> u8 {
+    let mut varint: [u8; 16] = [0; 16];
+    let mut pos = varint.len() - 1;
+
+    varint[pos] = (value & 127) as u8;
+
+    let mut value = value >> 7;
+    while value != 0 {
+        pos -= 1;
+        value -= 1;
+        varint[pos] = 128 | (value & 127) as u8;
+        value >>= 7;
+    }
+
+    if !buf.is_null() {
+        std::ptr::copy_nonoverlapping(varint.as_ptr().add(pos), buf, varint.len() - pos);
+    }
+
+    (varint.len() - pos) as u8
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_decode_varint() {
+        unsafe {
+            assert_eq!(decode_varint(&mut [0x00].as_slice().as_ptr()), 0);
+            assert_eq!(decode_varint(&mut [0x01].as_slice().as_ptr()), 1);
+            assert_eq!(decode_varint(&mut [0x7f].as_slice().as_ptr()), 127);
+            assert_eq!(decode_varint(&mut [0x80, 0x00].as_slice().as_ptr()), 128);
+            assert_eq!(decode_varint(&mut [0x80, 0x01].as_slice().as_ptr()), 129);
+            assert_eq!(decode_varint(&mut [0x80, 0x7f].as_slice().as_ptr()), 255);
+
+            // Overflows are expected to return 0.
+            assert_eq!(decode_varint(&mut [0x88; 16].as_slice().as_ptr()), 0);
+        }
+    }
+
+    #[test]
+    fn test_encode_varint() {
+        unsafe {
+            let mut varint: [u8; 16] = [0; 16];
+
+            assert_eq!(encode_varint(0, std::ptr::null_mut()), 1);
+
+            assert_eq!(encode_varint(0, varint.as_mut_slice().as_mut_ptr()), 1);
+            assert_eq!(varint, [0; 16]);
+
+            assert_eq!(encode_varint(10, varint.as_mut_slice().as_mut_ptr()), 1);
+            assert_eq!(varint, [10, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]);
+
+            assert_eq!(encode_varint(127, varint.as_mut_slice().as_mut_ptr()), 1);
+            assert_eq!(varint, [127, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]);
+
+            assert_eq!(encode_varint(128, varint.as_mut_slice().as_mut_ptr()), 2);
+            assert_eq!(varint, [128, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]);
+
+            assert_eq!(encode_varint(129, varint.as_mut_slice().as_mut_ptr()), 2);
+            assert_eq!(varint, [128, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]);
+
+            assert_eq!(encode_varint(255, varint.as_mut_slice().as_mut_ptr()), 2);
+            assert_eq!(varint, [128, 127, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]);
+        }
+    }
+}
-- 
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