From 7bb49794297b46aa1c577e89da24d39b4348c6d4 Mon Sep 17 00:00:00 2001
From: batsumaru <>
Date: Sun, 9 Aug 2026 12:39:50 +0900
Subject: Added fp_t multiplication

---
 include/fixed_point.h      |  3 +++
 meson.build                |  2 +-
 src/fixed_point.cpp        |  9 +++++++++
 tests/test_fixed_point.cpp | 20 ++++++++++++++++++++
 4 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/include/fixed_point.h b/include/fixed_point.h
index 1bdc5df..ff8e2b3 100644
--- a/include/fixed_point.h
+++ b/include/fixed_point.h
@@ -5,6 +5,8 @@
 #include <type_traits>
 
 struct fp_t {
+    static constexpr int32_t Q = 16;
+    static constexpr int32_t K = 1 << (Q - 1);
     std::int32_t raw;
     
     static constexpr fp_t from_raw(std::int32_t r) { return fp_t{r}; }
@@ -12,6 +14,7 @@ struct fp_t {
 
     fp_t operator+(const fp_t& other) const;
     fp_t operator-(const fp_t& other) const;
+    fp_t operator*(const fp_t& other) const;
     bool operator==(const fp_t& other) const;
 };
 
diff --git a/meson.build b/meson.build
index e5c654d..bff85ce 100644
--- a/meson.build
+++ b/meson.build
@@ -28,4 +28,4 @@ test_exe = executable('fg-tests',
   dependencies: [sim_core_dep, catch2_dep],
 )
 
-test('unit tests', test_exe)
+test('unit tests', test_exe, args: ['-r', 'automake'])
diff --git a/src/fixed_point.cpp b/src/fixed_point.cpp
index 9b2d8e2..847a5f7 100644
--- a/src/fixed_point.cpp
+++ b/src/fixed_point.cpp
@@ -8,6 +8,15 @@ fp_t fp_t::operator-(const fp_t& other) const {
     return {(int32_t)((uint32_t)raw - (uint32_t)other.raw)};
 }
 
+fp_t fp_t::operator*(const fp_t& other) const {
+    int32_t result;
+    int64_t temp;
+    temp = (int64_t)raw * (int64_t)other.raw;
+    temp += fp_t::K;
+    result = (int32_t)(temp >> fp_t::Q);
+    return {result};
+}
+
 bool fp_t::operator==(const fp_t& other) const {
     return raw == other.raw;
 }
diff --git a/tests/test_fixed_point.cpp b/tests/test_fixed_point.cpp
index b116340..6d1a594 100644
--- a/tests/test_fixed_point.cpp
+++ b/tests/test_fixed_point.cpp
@@ -17,6 +17,13 @@ TEST_CASE("fp_t subtraction anti-commutative") {
     REQUIRE(a.raw != 0);
 }
 
+TEST_CASE("fp_t multiplication is commutative") {
+    fp_t a = fp_t::from_int(2);
+    fp_t b = fp_t::from_int(5);
+    CHECK(a * b == b * a);
+    REQUIRE(a.raw != 1);
+}
+
 TEST_CASE("fp_t addition overflows") {
     fp_t a = fp_t::from_raw(INT32_MAX);
     fp_t b = fp_t::from_raw(1);
@@ -30,3 +37,16 @@ TEST_CASE("fp_t subtraction underflows") {
     fp_t c = fp_t::from_raw(INT32_MAX);
     CHECK(a - b == c);
 }
+
+TEST_CASE("fp_t multiplication overflows") {
+    fp_t a = fp_t::from_raw(INT32_MAX);
+    fp_t b = fp_t::from_int(2);
+    fp_t c = fp_t::from_raw(-2);
+    CHECK(a * b == c);
+}
+
+TEST_CASE("fp_t multiplication drops percision under 1 ULP") {
+    fp_t a = fp_t::from_raw(1);
+    fp_t b = fp_t::from_raw(0);
+    CHECK(a * a == b);
+}
-- 
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