From bccd111a16ac34e01edd13e88876f0371193d294 Mon Sep 17 00:00:00 2001
From: Hojun-Cho <ghwns6743@gmail.com>
Date: Sat, 13 Jun 2026 10:36:11 +0900
Subject: [PATCH] lib/fmt: encode rune args as UTF-8, not a truncated low byte
 (#66)
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writeone and formatraw's rune arm did `buf[0] = r: u8; putbytes(...,1)`,
emitting only the low byte — invalid UTF-8 for any rune > 0x7F, with a
success return. rawlen's rune arm hardcoded 1, desyncing width-padding
for multibyte runes. Route both write arms through utf8.encoderune and
rawlen through utf8.runesz (ref/hare/fmt/print.ha:84). Uses the current
in-tree encoderune(out: []u8, r) caller-buffer signature; report-#168's
signature realignment is a separate item, not folded here.

fmttest gains fprintrune (1/2/3/4-byte runes) + fprintf_rune_width
(multibyte pad).
---
 lib/fmt/fmt.ww     | 25 ++++++++++++++++++++-----
 lib/fmt/fmttest.ww | 40 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 60 insertions(+), 5 deletions(-)

diff --git a/lib/fmt/fmt.ww b/lib/fmt/fmt.ww
index 1cbdd99f..9a0d33b8 100644
--- a/lib/fmt/fmt.ww
+++ b/lib/fmt/fmt.ww
@@ -22,6 +22,7 @@
 package fmt;
 
 import io;
+import encoding.utf8;
 import math;
 import memio;
 import os;
@@ -115,9 +116,15 @@ fn writeone(s: io.handle, a: formattable) (size | io.error) = {
 		return putbytes(s, v.ptr, v.len);
 	};
 	case let r: rune => {
+		// ref/hare/fmt/print.ha:84 io::write(out, utf8::encoderune(r)):
+		// emit the full UTF-8 encoding, not the truncated low byte.
 		let buf: [4]u8;
-		buf[0] = r: u8;
-		return putbytes(s, &buf[0], 1);
+		let sl: []u8;
+		sl.ptr = &buf[0];
+		sl.len = 4;
+		sl.cap = 4;
+		let nn: i32 = utf8.encoderune(sl, r);
+		return putbytes(s, &buf[0], nn);
 	};
 	case let v: f64 => {
 		// strconv.f64tos static-buffer view consumed before next
@@ -348,7 +355,10 @@ fn rawlen(arg: formattable, m: *mods) i32 = {
 	case let v: i64  => return rawleni64(v, m);
 	case let v: str  => return rawlenstr(v, m);
 	case let b: bool => { if (b) { return 4; }; return 5; };
-	case let r: rune => return 1;
+	// width-padding must count the rune's encoded byte length, else a
+	// multibyte rune desyncs the pad (ref/hare/fmt/print.ha:84 emits the
+	// full encoding; runesz is its length).
+	case let r: rune => return utf8.runesz(r);
 	case let v: f64  => return rawlenf64(v, m);
 	case let v: int  => return rawleni64(v: i64, m);
 	case let v: uint => return rawlenu64(v: u64, m);
@@ -412,9 +422,14 @@ fn formatraw(s: io.handle, arg: formattable, m: *mods) (size | io.error) = {
 		return putbytes(s, v.ptr, v.len);
 	};
 	case let r: rune => {
+		// ref/hare/fmt/print.ha:84: full UTF-8 encoding, not the low byte.
 		let buf: [4]u8;
-		buf[0] = r: u8;
-		return putbytes(s, &buf[0], 1);
+		let sl: []u8;
+		sl.ptr = &buf[0];
+		sl.len = 4;
+		sl.cap = 4;
+		let nn: i32 = utf8.encoderune(sl, r);
+		return putbytes(s, &buf[0], nn);
 	};
 	case let v: f64 => {
 		// strconv.f64tos prepends '-' for negative values; peel here so
diff --git a/lib/fmt/fmttest.ww b/lib/fmt/fmttest.ww
index e60f2a66..3766bc8d 100644
--- a/lib/fmt/fmttest.ww
+++ b/lib/fmt/fmttest.ww
@@ -303,6 +303,46 @@ fn errsource() io.stream = {
 	match (c) { case void => {}; case let eioe: io.error => abort(); };
 };
 
+// ---- fprint: rune args emit full UTF-8, not a truncated byte (#66) -----
+// writeone/formatraw's rune arm did `buf[0] = r: u8; putbytes(...,1)`,
+// emitting only the low byte (invalid UTF-8 for r > 0x7F). The fix routes
+// through utf8.encoderune (ref/hare/fmt/print.ha:84). Edge runes: é (2B),
+// € (3B), 😀 (4B), A (1B).
+
+@test fn fprintrune() void = {
+	let mem: memio.stream = memio.dynamic();
+	let s: io.stream = &mem.vt;
+
+	let r: (size | io.error) = fmt.fprint(s, 233: rune, 0x20AC: rune,
+	    0x1F600: rune, 65: rune);
+	match (r) {
+	case let n: size => { assert(!(n: i32 != 13)); };	// 2+1+3+1+4+1+1
+	case let eioe: io.error => abort();
+	};
+	assert(!(!streq(memio.string(&mem), "é € 😀 A")));
+
+	let c: (void | io.error) = io.close(s);
+	match (c) { case void => {}; case let eioe: io.error => abort(); };
+};
+
+// rawlen's rune arm hardcoded 1, desyncing width-padding for a multibyte
+// rune. With runesz it counts the encoded length: € (3B) in width 5 pads
+// to "  €" (2 spaces + 3 bytes = 5).
+@test fn fprintf_rune_width() void = {
+	let mem: memio.stream = memio.dynamic();
+	let s: io.stream = &mem.vt;
+
+	let r: (size | io.error) = fmt.fprintf(s, "{:5}", 0x20AC: rune);
+	match (r) {
+	case let n: size => { assert(!(n: i32 != 5)); };
+	case let eioe: io.error => abort();
+	};
+	assert(!(!streq(memio.string(&mem), "  €")));
+
+	let c: (void | io.error) = io.close(s);
+	match (c) { case void => {}; case let eioe: io.error => abort(); };
+};
+
 @test fn fprintf_pad_underscore() void = {
 	let mem: memio.stream = memio.dynamic();
 	let s: io.stream = &mem.vt;