From d79dd55a18cb158778fbdc796a60f27518d1f971 Mon Sep 17 00:00:00 2001
From: "B. Watson" <urchlay@slackware.uk>
Date: Tue, 18 Jun 2024 06:53:54 -0400
Subject: cxrefbas and renumbas: round fractional line numbers like BASIC does.

---
 bcdfp.c      | 32 +++++++++++++++++++++++++-------
 cxrefbas.rst |  5 +++++
 linetab.c    | 12 ++++++++++++
 renumbas.rst |  5 +++++
 4 files changed, 47 insertions(+), 7 deletions(-)

diff --git a/bcdfp.c b/bcdfp.c
index 76157c1..a80e55d 100644
--- a/bcdfp.c
+++ b/bcdfp.c
@@ -1,8 +1,18 @@
 #include <string.h>
+#include <stdio.h>
 #include "bcdfp.h"
 
+/* very dumb and limited BCD floating point conversions.
+   they're written this way because they're only required to
+   support line numbers, and I don't want to have to link with
+   the math library (-lm). */
+
 extern void die(const char *msg);
 
+void die_range(void) {
+	die("Line number out of range (>65535)");
+}
+
 unsigned char bcd2int(unsigned char bcd) {
 	return (bcd >> 4) * 10 + (bcd & 0x0f);
 }
@@ -12,24 +22,32 @@ unsigned char int2bcd(unsigned char i) {
 }
 
 unsigned short fp2int(const unsigned char *fp) {
-	unsigned short result = 0;
+	unsigned int result = 0;
 
 	/* examine the exponent/sign byte */
 	if(fp[0] == 0) return 0; /* special case */
-	if(fp[0] & 0x80) die("negative numbers not supported");
+	if(fp[0] & 0x80) die("Negative line numbers not supported");
 
 	switch(fp[0]) {
 		case 0x40:
-			result = bcd2int(fp[1]); break;
+			result = bcd2int(fp[1]);
+			if(fp[2] >= 0x50) result++;
+			break;
 		case 0x41:
-			result = bcd2int(fp[1]) * 100 + bcd2int(fp[2]); break;
+			result = bcd2int(fp[1]) * 100 + bcd2int(fp[2]);
+			if(fp[3] >= 0x50) result++;
+			break;
 		case 0x42:
-			result = bcd2int(fp[1]) * 10000 + bcd2int(fp[2]) * 100 + bcd2int(fp[3]); break;
+			result = bcd2int(fp[1]) * 10000 + bcd2int(fp[2]) * 100 + bcd2int(fp[3]);
+			if(fp[4] >= 0x50) result++;
+			break;
 		default:
-			die("number out of range"); break;
+			die_range(); break;
 	}
 
-	return result;
+	if(result > 0xffff) die_range();
+
+	return (unsigned short)result;
 }
 
 void int2fp(unsigned short num, unsigned char *fp) {
diff --git a/cxrefbas.rst b/cxrefbas.rst
index d80d818..7004217 100644
--- a/cxrefbas.rst
+++ b/cxrefbas.rst
@@ -58,6 +58,11 @@ warning on standard error, e.g. *GOTO A* or *GOSUB 100\*A*. Even *GOTO
 
 Line numbers above 32767, e.g. *TRAP 40000*, are not listed.
 
+Atari BASIC allows fractional line numbers, such as *GOTO 123.4*.
+These are rounded to the nearest integer when the program is
+executed. **cxrefbas** handles these correctly, although you're
+not likely to run into them in real-world programs.
+
 OPTIONS
 =======
 
diff --git a/linetab.c b/linetab.c
index 0351f59..59a96e3 100644
--- a/linetab.c
+++ b/linetab.c
@@ -93,6 +93,8 @@ void computed_msg(unsigned short lineno) {
 
 CALLBACK(got_var) {
 	switch(last_cmd) {
+		/* any use of a variable in the arguments to these means
+		   we can't renumber that argument. */
 		case CMD_GOTO:
 		case CMD_GO_TO:
 		case CMD_GOSUB:
@@ -102,6 +104,7 @@ CALLBACK(got_var) {
 			computed_msg(lineno);
 			break;
 		case CMD_ON:
+			/* vars are OK in ON, before the GOTO or GOSUB */
 			if(on_op) computed_msg(lineno);
 			break;
 		default:
@@ -119,9 +122,14 @@ CALLBACK(got_exp) {
 	}
 
 	if(tok != OP_NUMCONST) return;
+
+	/* beware: standalone only means nothing *follows* the constant
+	   in the same expression. still have to check last_tok to see
+	   what came before. */
 	standalone = is_standalone_num(pos);
 
 	switch(last_cmd) {
+		/* these take a single argument */
 		case CMD_GOTO:
 		case CMD_GO_TO:
 		case CMD_GOSUB:
@@ -134,11 +142,14 @@ CALLBACK(got_exp) {
 			}
 			break;
 		case CMD_IF:
+			/* this only applies to bare line numbers, like IF A THEN 1000,
+			   not IF A THEN GOTO 1000 (or anything else after THEN). */
 			if(last_tok == OP_THEN) {
 				add_lineref(lineno, pos + 1);
 			}
 			break;
 		case CMD_ON: {
+				/* takes arbitrary number of arguments */
 				switch(last_tok) {
 					case OP_GOTO:
 					case OP_GOSUB:
@@ -154,6 +165,7 @@ CALLBACK(got_exp) {
 			}
 			break;
 		case CMD_LIST: {
+				/* takes one or two arguments */
 				switch(last_tok) {
 					case CMD_LIST:
 					case OP_COMMA:
diff --git a/renumbas.rst b/renumbas.rst
index d4b0990..7cef711 100644
--- a/renumbas.rst
+++ b/renumbas.rst
@@ -44,6 +44,11 @@ Remember that the maximum line number for Atari BASIC is 32767.
 Renumbering will fail, if the chosen start and increment values
 would result in lines with numbers higher than this.
 
+Atari BASIC allows fractional line numbers, such as *GOTO 123.4*.
+These are rounded to the nearest integer when the program is
+executed. **renumbas** handles these correctly, although you're
+not likely to run into them in real-world programs.
+
 OPTIONS
 =======
 
-- 
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