strconv: bug fix for "${239.5:0.0f}" (#13483)

vlib/builtin/float_test.v

@@ -145,3 +145,86 @@ fn test_f32_eq_epsilon() {
 	assert !e.eq_epsilon(y)
 	assert !(-y).eq_epsilon(-e)
 }
+
+fn test_float_point_formatting_rounding() {
+	float_1 := 462.18
+	float_2 := 45.02227
+	float_3 := 238.5
+	float_4 := 239.5
+
+	assert '${float_1:0.0f}' == '462'
+	assert '${float_2:0.0f}' == '45'
+	assert '${float_3:0.0f}' == '239'
+	assert '${float_4:0.0f}' == '240'
+	//
+	//	
+	assert '${239.5555551:0.0f}' == '240'
+	assert '${239.5555551:0.1f}' == '239.6'
+	assert '${239.5555551:0.2f}' == '239.56'
+	assert '${239.5555551:0.3f}' == '239.556'
+	assert '${239.5555551:0.4f}' == '239.5556'
+	assert '${239.5555551:0.5f}' == '239.55556'
+	assert '${239.5555551:0.6f}' == '239.555555'
+	assert '${239.5555551:0.7f}' == '239.5555551'
+	assert '${239.5555551:0.8f}' == '239.55555510'
+	assert '${239.5555551:0.9f}' == '239.555555100'
+	assert '${239.5555551:0.10f}' == '239.5555551000'
+	assert '${239.5555551:0.11f}' == '239.55555510000'
+	//
+	assert '${239.5:0.0f}' == '240'
+	assert '${239.55:0.1f}' == '239.6'
+	assert '${239.555:0.2f}' == '239.56'
+	assert '${239.5555:0.3f}' == '239.555' // NB: 5 ?
+	assert '${239.55555:0.4f}' == '239.5556'
+	assert '${239.555555:0.5f}' == '239.55555' // NB: 5 ?
+	assert '${239.5555555:0.6f}' == '239.555556' // after this, it is all ending in 6
+	assert '${239.55555555:0.7f}' == '239.5555556'
+	assert '${239.555555555:0.8f}' == '239.55555556'
+	assert '${239.5555555555:0.9f}' == '239.555555556'
+	assert '${239.55555555555:0.10f}' == '239.5555555556'
+	//
+	assert '${239.5550:0.3f}' == '239.555'
+	assert '${239.5551:0.3f}' == '239.555'
+	assert '${239.5552:0.3f}' == '239.555'
+	assert '${239.5553:0.3f}' == '239.555'
+	assert '${239.5554:0.3f}' == '239.555'
+	assert '${239.5555:0.3f}' == '239.555'
+	assert '${239.5556:0.3f}' == '239.556' // rounding at last 6 ?
+	assert '${239.5557:0.3f}' == '239.556'
+	assert '${239.5558:0.3f}' == '239.556'
+	assert '${239.5559:0.3f}' == '239.556'
+	//
+	assert '${239.5555551:0.6f}' == '239.555555'
+	assert '${239.5555552:0.6f}' == '239.555555'
+	assert '${239.5555553:0.6f}' == '239.555555'
+	assert '${239.5555554:0.6f}' == '239.555555'
+	assert '${239.5555555:0.6f}' == '239.555556'
+	assert '${239.5555556:0.6f}' == '239.555556'
+	assert '${239.5555557:0.6f}' == '239.555556'
+	assert '${239.5555558:0.6f}' == '239.555556'
+	assert '${239.5555559:0.6f}' == '239.555556'
+	//
+	assert '${239.55555555555:0.10f}' == '239.5555555556'
+	assert '${239.55555555555:0.9f}' == '239.555555556'
+	assert '${239.55555555555:0.8f}' == '239.55555556'
+	assert '${239.55555555555:0.7f}' == '239.5555556'
+	assert '${239.55555555555:0.6f}' == '239.555556'
+	assert '${239.55555555555:0.5f}' == '239.55556'
+	assert '${239.55555555555:0.4f}' == '239.5556'
+	assert '${239.55555555555:0.3f}' == '239.556'
+	assert '${239.55555555555:0.2f}' == '239.56'
+	assert '${239.55555555555:0.1f}' == '239.6'
+	assert '${239.55555555555:0.0f}' == '240'
+	//
+	assert '${-239.55555555555:0.10f}' == '-239.5555555556'
+	assert '${-239.55555555555:0.9f}' == '-239.555555556'
+	assert '${-239.55555555555:0.8f}' == '-239.55555556'
+	assert '${-239.55555555555:0.7f}' == '-239.5555556'
+	assert '${-239.55555555555:0.6f}' == '-239.555556'
+	assert '${-239.55555555555:0.5f}' == '-239.55556'
+	assert '${-239.55555555555:0.4f}' == '-239.5556'
+	assert '${-239.55555555555:0.3f}' == '-239.556'
+	assert '${-239.55555555555:0.2f}' == '-239.56'
+	assert '${-239.55555555555:0.1f}' == '-239.6'
+	assert '${-239.55555555555:0.0f}' == '-240'
+}

vlib/strconv/format_mem.c.v

@@ -156,7 +156,7 @@ pub fn f64_to_str_lnd1(f f64, dec_digit int) string {
 
 		mut dot_res_sp := -1
 
-		// get sign and deciaml parts
+		// get sign and decimal parts
 		for c in s {
 			if c == `-` {
 				sgn = -1
@@ -263,6 +263,10 @@ pub fn f64_to_str_lnd1(f f64, dec_digit int) string {
 
 		// no more digits needed, stop here
 		if dec_digit <= 0 {
+			// C.printf(c'f: %f, i: %d, res.data: %p | dot_res_sp: %d | *(res.data): %s \n', f, i, res.data, dot_res_sp, res.data)
+			if dot_res_sp < 0 {
+				dot_res_sp = i + 1
+			}
 			tmp_res := tos(res.data, dot_res_sp).clone()
 			res.free()
 			return tmp_res