varint: reimplement as test balloon for Rust

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 <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 94 insertions, 0 deletions

diff --git a/src/lib.rs b/src/lib.rs
index e69de29bb2..9da70d8b57 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -0,0 +1 @@
+pub mod varint;
diff --git a/src/meson.build b/src/meson.build
index c8d874b210..25b9ad5a14 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -1,5 +1,6 @@
 libgit_rs_sources = [
   'lib.rs',
+  'varint.rs',
 ]
 
 # Unfortunately we must use a wrapper command to move the output file into the
diff --git a/src/varint.rs b/src/varint.rs
new file mode 100644
index 0000000000..6e610bdd8e
--- /dev/null
+++ b/src/varint.rs
@@ -0,0 +1,92 @@
+#[no_mangle]
+pub unsafe extern "C" fn decode_varint(bufp: *mut *const u8) -> u64 {
+    let mut buf = *bufp;
+    let mut c = *buf;
+    let mut val = u64::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) + u64::from(c & 127);
+    }
+
+    *bufp = buf;
+    val
+}
+
+#[no_mangle]
+pub unsafe extern "C" fn encode_varint(value: u64, 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]);
+        }
+    }
+}