From 09fab30bcbf3d8aa074096839061d614037dc921 Mon Sep 17 00:00:00 2001
From: aSumo <1xtelescope@gmail.com>
Date: Thu, 3 Sep 2026 21:01:13 +0900
Subject: [PATCH] Make Serial.print(foo, HEX) ALWAYS print 2 chars per byte

---
 api/Print.cpp                 | 22 +++++++++++++++++++---
 test/src/Print/test_print.cpp | 12 ++++++------
 2 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/api/Print.cpp b/api/Print.cpp
index 4a6e942a..5c6cc845 100644
--- a/api/Print.cpp
+++ b/api/Print.cpp
@@ -242,11 +242,16 @@ size_t Print::printNumber(unsigned long n, uint8_t base)
 {
   char buf[8 * sizeof(long) + 1]; // Assumes 8-bit chars plus zero byte.
   char *str = &buf[sizeof(buf) - 1];
-
   *str = '\0';
+  size_t leadingZero = 0;
 
   // prevent crash if called with base == 1
   if (base < 2) base = 10;
+  // insert leading zero for hex values less than 0x10
+  else if (base == 16 && n < 16) {
+    write('0');
+    leadingZero = 1;
+  }
 
   do {
     char c = n % base;
@@ -255,7 +260,7 @@ size_t Print::printNumber(unsigned long n, uint8_t base)
     *--str = c < 10 ? c + '0' : c + 'A' - 10;
   } while(n);
 
-  return write(str);
+  return write(str) + leadingZero;
 }
 
 // REFERENCE IMPLEMENTATION FOR ULL
@@ -287,15 +292,26 @@ size_t Print::printULLNumber(unsigned long long n64, uint8_t base)
   char buf[64];
   uint8_t i = 0;
   uint8_t innerLoops = 0;
+  size_t leadingZero = 0;
 
   // Special case workaround https://github.com/arduino/ArduinoCore-API/issues/178
   if (n64 == 0) {
+    if (base == 16) {
+      write('0');
+      write('0');
+      return 2;
+    }
     write('0');
     return 1;
   }
 
   // prevent crash if called with base == 1
   if (base < 2) base = 10;
+  // insert leading zero for hex values less than 0x10
+  else if (base == 16 && n64 < 16) {
+    write('0');
+    leadingZero = 1;
+  }
 
   // process chunks that fit in "16 bit math".
   uint16_t top = 0xFFFF / base;
@@ -336,7 +352,7 @@ size_t Print::printULLNumber(unsigned long long n64, uint8_t base)
     '0' + buf[i - 1] :
     'A' + buf[i - 1] - 10));
 
-  return bytes;
+  return bytes + leadingZero;
 }
 
 size_t Print::printFloat(double number, int digits)
diff --git a/test/src/Print/test_print.cpp b/test/src/Print/test_print.cpp
index 1a41c462..b92f530e 100644
--- a/test/src/Print/test_print.cpp
+++ b/test/src/Print/test_print.cpp
@@ -118,18 +118,18 @@ TEST_CASE ("Print::print(unsigned long long, int = DEC|HEX|OCT|BIN)", "[Print-pr
     unsigned long long const val = 0;
 
     WHEN("DEC") { mock.print(val, DEC); REQUIRE(mock._str  == "0"); }
-    WHEN("HEX") { mock.print(val, HEX); REQUIRE(mock._str  == "0"); }
+    WHEN("HEX") { mock.print(val, HEX); REQUIRE(mock._str  == "00"); }
     WHEN("OCT") { mock.print(val, OCT); REQUIRE(mock._str  == "0"); }
     WHEN("BIN") { mock.print(val, BIN); REQUIRE(mock._str  == "0"); }
   }
   GIVEN("a non-zero value ...")
   {
-    unsigned long long const val = 17;
+    unsigned long long const val = 15;
 
-    WHEN("DEC") { mock.print(val, DEC); REQUIRE(mock._str  == "17"); }
-    WHEN("HEX") { mock.print(val, HEX); REQUIRE(mock._str  == "11"); }
-    WHEN("OCT") { mock.print(val, OCT); REQUIRE(mock._str  == "21"); }
-    WHEN("BIN") { mock.print(val, BIN); REQUIRE(mock._str  == "10001"); }
+    WHEN("DEC") { mock.print(val, DEC); REQUIRE(mock._str  == "15"); }
+    WHEN("HEX") { mock.print(val, HEX); REQUIRE(mock._str  == "0F"); }
+    WHEN("OCT") { mock.print(val, OCT); REQUIRE(mock._str  == "17"); }
+    WHEN("BIN") { mock.print(val, BIN); REQUIRE(mock._str  == "1111"); }
   }
 }