From 1185d2f52957d4d754173d71fa06e15338f08cec Mon Sep 17 00:00:00 2001
From: Ethan Sarp <11684270+esarp@users.noreply.github.com>
Date: Tue, 9 Dec 2025 12:36:09 -0600
Subject: [PATCH 1/3] Implement bytes.startswith in mypyc

---
 mypyc/lib-rt/CPy.h                 |  2 +-
 mypyc/lib-rt/bytes_ops.c           | 39 ++++++++++++++++++++++++++++++
 mypyc/primitives/bytes_ops.py      | 11 +++++++++
 mypyc/test-data/fixtures/ir.py     |  1 +
 mypyc/test-data/irbuild-bytes.test | 14 +++++++++++
 mypyc/test-data/run-bytes.test     | 13 ++++++++++
 6 files changed, 79 insertions(+), 1 deletion(-)

diff --git a/mypyc/lib-rt/CPy.h b/mypyc/lib-rt/CPy.h
index 5526508f5aca..4374fed04921 100644
--- a/mypyc/lib-rt/CPy.h
+++ b/mypyc/lib-rt/CPy.h
@@ -783,7 +783,7 @@ PyObject *CPyBytes_Concat(PyObject *a, PyObject *b);
 PyObject *CPyBytes_Join(PyObject *sep, PyObject *iter);
 CPyTagged CPyBytes_Ord(PyObject *obj);
 PyObject *CPyBytes_Multiply(PyObject *bytes, CPyTagged count);
-
+int CPyBytes_Startswith(PyObject *self, PyObject *subobj);
 
 int CPyBytes_Compare(PyObject *left, PyObject *right);
 
diff --git a/mypyc/lib-rt/bytes_ops.c b/mypyc/lib-rt/bytes_ops.c
index 8ecf9337c28b..05a14aecf47b 100644
--- a/mypyc/lib-rt/bytes_ops.c
+++ b/mypyc/lib-rt/bytes_ops.c
@@ -171,3 +171,42 @@ PyObject *CPyBytes_Multiply(PyObject *bytes, CPyTagged count) {
     }
     return PySequence_Repeat(bytes, temp_count);
 }
+
+int CPyBytes_Startswith(PyObject *self, PyObject *subobj) {
+    if (PyBytes_CheckExact(self) && PyBytes_CheckExact(subobj)) {
+        if (self == subobj) {
+            return 1;
+        }
+
+        Py_ssize_t self_len = PyBytes_GET_SIZE(self);
+        Py_ssize_t subobj_len = PyBytes_GET_SIZE(subobj);
+
+        if (subobj_len > self_len) {
+            return 0;
+        }
+
+        const char *self_buf = PyBytes_AS_STRING(self);
+        const char *subobj_buf = PyBytes_AS_STRING(subobj);
+
+        if (subobj_len == 0) {
+            return 1;
+        }
+
+        return memcmp(self_buf, subobj_buf, (size_t)subobj_len) == 0 ? 1 : 0;
+    }
+    _Py_IDENTIFIER(startswith);
+    PyObject *name = _PyUnicode_FromId(&PyId_startswith);
+    if (name == NULL) {
+        return 2;
+    }
+    PyObject *result = PyObject_CallMethodOneArg(self, name, subobj);
+    if (result == NULL) {
+        return 2;
+    }
+    int ret = PyObject_IsTrue(result);
+    Py_DECREF(result);
+    if (ret < 0) {
+        return 2;
+    }
+    return ret;
+}
diff --git a/mypyc/primitives/bytes_ops.py b/mypyc/primitives/bytes_ops.py
index 0669ddac00df..728da4181135 100644
--- a/mypyc/primitives/bytes_ops.py
+++ b/mypyc/primitives/bytes_ops.py
@@ -7,6 +7,7 @@
 from mypyc.ir.rtypes import (
     RUnion,
     bit_rprimitive,
+    bool_rprimitive,
     bytes_rprimitive,
     c_int_rprimitive,
     c_pyssize_t_rprimitive,
@@ -139,6 +140,16 @@
     dependencies=[BYTES_EXTRA_OPS],
 )
 
+# bytes.startswith(bytes)
+method_op(
+    name="startswith",
+    arg_types=[bytes_rprimitive, bytes_rprimitive],
+    return_type=c_int_rprimitive,
+    c_function_name="CPyBytes_Startswith",
+    truncated_type=bool_rprimitive,
+    error_kind=ERR_MAGIC,
+)
+
 # Join bytes objects and return a new bytes.
 # The first argument is the total number of the following bytes.
 bytes_build_op = custom_op(
diff --git a/mypyc/test-data/fixtures/ir.py b/mypyc/test-data/fixtures/ir.py
index d9202707124b..498e59ce46e3 100644
--- a/mypyc/test-data/fixtures/ir.py
+++ b/mypyc/test-data/fixtures/ir.py
@@ -179,6 +179,7 @@ def __getitem__(self, i: slice) -> bytes: ...
     def join(self, x: Iterable[object]) -> bytes: ...
     def decode(self, encoding: str=..., errors: str=...) -> str: ...
     def translate(self, t: bytes) -> bytes: ...
+    def startswith(self, t: bytes) -> bool: ...
     def __iter__(self) -> Iterator[int]: ...
 
 class bytearray:
diff --git a/mypyc/test-data/irbuild-bytes.test b/mypyc/test-data/irbuild-bytes.test
index 9473944f44fe..780c474e3614 100644
--- a/mypyc/test-data/irbuild-bytes.test
+++ b/mypyc/test-data/irbuild-bytes.test
@@ -248,3 +248,17 @@ def f(b, table):
 L0:
     r0 = CPyBytes_Translate(b, table)
     return r0
+
+[case testBytesStartsWith]
+def f(a: bytes, b: bytes) -> bool:
+    return a.startswith(b)
+[out]
+def f(a, b):
+    a, b :: bytes
+    r0 :: i32
+    r1 :: bool
+L0:
+    r0 = CPyBytes_Startswith(a, b)
+    r1 = truncate r0: i32 to builtins.bool
+    return r1
+
diff --git a/mypyc/test-data/run-bytes.test b/mypyc/test-data/run-bytes.test
index 9a319b636772..2891a62452da 100644
--- a/mypyc/test-data/run-bytes.test
+++ b/mypyc/test-data/run-bytes.test
@@ -200,6 +200,19 @@ def test_translate() -> None:
     with assertRaises(ValueError, "translation table must be 256 characters long"):
         b'test'.translate(bytes(100))
 
+def test_startswith() -> None:
+    # Test default behavior
+    test = b'some string'
+    assert test.startswith(b'some')
+    assert test.startswith(b'some string')
+    assert not test.startswith(b'other')
+    assert not test.startswith(b'some string but longer')
+
+    # Test empty cases
+    assert test.startswith(b'')
+    assert b''.startswith(b'')
+    assert not b''.startswith(test)
+
 [case testBytesSlicing]
 def test_bytes_slicing() -> None:
     b = b'abcdefg'

From 5dbd6a958b877a9e7edfb6c6eb8a26d02519c596 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Tue, 9 Dec 2025 18:52:19 +0000
Subject: [PATCH 2/3] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 mypyc/test-data/irbuild-bytes.test | 1 -
 1 file changed, 1 deletion(-)

diff --git a/mypyc/test-data/irbuild-bytes.test b/mypyc/test-data/irbuild-bytes.test
index 780c474e3614..5e7c546eb25a 100644
--- a/mypyc/test-data/irbuild-bytes.test
+++ b/mypyc/test-data/irbuild-bytes.test
@@ -261,4 +261,3 @@ L0:
     r0 = CPyBytes_Startswith(a, b)
     r1 = truncate r0: i32 to builtins.bool
     return r1
-

From 69fbc684df64fbb7fea0ecdbda636ff6c24302bd Mon Sep 17 00:00:00 2001
From: Ethan Sarp <11684270+esarp@users.noreply.github.com>
Date: Wed, 10 Dec 2025 11:36:31 -0600
Subject: [PATCH 3/3] Optimize checks; add bytearray tests

---
 mypyc/lib-rt/bytes_ops.c       | 9 ++++-----
 mypyc/test-data/fixtures/ir.py | 1 +
 mypyc/test-data/run-bytes.test | 8 ++++++++
 3 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/mypyc/lib-rt/bytes_ops.c b/mypyc/lib-rt/bytes_ops.c
index 05a14aecf47b..25718b4603b3 100644
--- a/mypyc/lib-rt/bytes_ops.c
+++ b/mypyc/lib-rt/bytes_ops.c
@@ -178,9 +178,12 @@ int CPyBytes_Startswith(PyObject *self, PyObject *subobj) {
             return 1;
         }
 
-        Py_ssize_t self_len = PyBytes_GET_SIZE(self);
         Py_ssize_t subobj_len = PyBytes_GET_SIZE(subobj);
+        if (subobj_len == 0) {
+            return 1;
+        }
 
+        Py_ssize_t self_len = PyBytes_GET_SIZE(self);
         if (subobj_len > self_len) {
             return 0;
         }
@@ -188,10 +191,6 @@ int CPyBytes_Startswith(PyObject *self, PyObject *subobj) {
         const char *self_buf = PyBytes_AS_STRING(self);
         const char *subobj_buf = PyBytes_AS_STRING(subobj);
 
-        if (subobj_len == 0) {
-            return 1;
-        }
-
         return memcmp(self_buf, subobj_buf, (size_t)subobj_len) == 0 ? 1 : 0;
     }
     _Py_IDENTIFIER(startswith);
diff --git a/mypyc/test-data/fixtures/ir.py b/mypyc/test-data/fixtures/ir.py
index 498e59ce46e3..592f6676e95e 100644
--- a/mypyc/test-data/fixtures/ir.py
+++ b/mypyc/test-data/fixtures/ir.py
@@ -193,6 +193,7 @@ def __add__(self, s: bytes) -> bytearray: ...
     def __setitem__(self, i: int, o: int) -> None: ...
     def __getitem__(self, i: int) -> int: ...
     def decode(self, x: str = ..., y: str = ...) -> str: ...
+    def startswith(self, t: bytes) -> bool: ...
 
 class bool(int):
     def __init__(self, o: object = ...) -> None: ...
diff --git a/mypyc/test-data/run-bytes.test b/mypyc/test-data/run-bytes.test
index 2891a62452da..6e4b57152a4b 100644
--- a/mypyc/test-data/run-bytes.test
+++ b/mypyc/test-data/run-bytes.test
@@ -213,6 +213,14 @@ def test_startswith() -> None:
     assert b''.startswith(b'')
     assert not b''.startswith(test)
 
+    # Test bytearray to verify slow paths
+    assert test.startswith(bytearray(b'some'))
+    assert not test.startswith(bytearray(b'other'))
+
+    test = bytearray(b'some string')
+    assert test.startswith(b'some')
+    assert not test.startswith(b'other')
+
 [case testBytesSlicing]
 def test_bytes_slicing() -> None:
     b = b'abcdefg'