From 9d4f68606c919791d44e86e7a50812c29be32fa3 Mon Sep 17 00:00:00 2001
From: batsumaru <>
Date: Sun, 9 Aug 2026 12:00:27 +0900
Subject: Add fp_t subtraction

---
 include/fixed_point.h      |  1 +
 src/fixed_point.cpp        |  4 ++++
 tests/test_fixed_point.cpp | 16 ++++++++++++++++
 3 files changed, 21 insertions(+)

diff --git a/include/fixed_point.h b/include/fixed_point.h
index 254bc91..1bdc5df 100644
--- a/include/fixed_point.h
+++ b/include/fixed_point.h
@@ -11,6 +11,7 @@ struct fp_t {
     static constexpr fp_t from_int(std::int32_t i) { return fp_t{i << 16}; }
 
     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/src/fixed_point.cpp b/src/fixed_point.cpp
index 7ff77a0..9b2d8e2 100644
--- a/src/fixed_point.cpp
+++ b/src/fixed_point.cpp
@@ -4,6 +4,10 @@ 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 {
+    return {(int32_t)((uint32_t)raw - (uint32_t)other.raw)};
+}
+
 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 b3d3e78..b116340 100644
--- a/tests/test_fixed_point.cpp
+++ b/tests/test_fixed_point.cpp
@@ -8,9 +8,25 @@ TEST_CASE("fp_t addition is commutative") {
     REQUIRE(a.raw != 0);
 }
 
+TEST_CASE("fp_t subtraction anti-commutative") {
+    fp_t a = fp_t::from_int(3);
+    fp_t b = fp_t::from_int(2);
+    fp_t c = fp_t::from_int(1);
+    fp_t d = fp_t::from_int(-1);
+    CHECK((a - b == c && b - a == d));
+    REQUIRE(a.raw != 0);
+}
+
 TEST_CASE("fp_t addition overflows") {
     fp_t a = fp_t::from_raw(INT32_MAX);
     fp_t b = fp_t::from_raw(1);
     fp_t c = fp_t::from_raw(INT32_MIN);
     CHECK(a + b == c);
 }
+
+TEST_CASE("fp_t subtraction underflows") {
+    fp_t a = fp_t::from_raw(INT32_MIN);
+    fp_t b = fp_t::from_raw(1);
+    fp_t c = fp_t::from_raw(INT32_MAX);
+    CHECK(a - b == c);
+}
-- 
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