From 84d9a7ce82b6184639caeeac7a4026818833d6ee Mon Sep 17 00:00:00 2001
From: Lawrence Kesteloot <lk@teamten.com>
Date: Sun, 5 Aug 2018 16:43:50 -0700
Subject: [PATCH] Add unary minus (negative).

---
 README.md  |  4 ++--
 exporter.h |  1 +
 exporter.s |  2 ++
 main.c     | 20 +++++++++++++++++---
 4 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/README.md b/README.md
index 0d7afe8..855ecd0 100644
--- a/README.md
+++ b/README.md
@@ -9,8 +9,8 @@ line numbers, 16-bit integer variables, `HOME`, `PRINT`, `IF/THEN`,
 `FOR/NEXT`, `GOTO`, low-res graphics (`GR`, `PLOT`, `COLOR=`, `TEXT`),
 `POKE`, and integer and boolean arithmetic.
 
-Not supported: Floating point, strings, negative
-integers, high-res graphics, `DATA/READ/RESUME`, `GOSUB/RETURN/POP`,
+Not supported: Floating point, strings,
+high-res graphics, `DATA/READ/RESUME`, `GOSUB/RETURN/POP`,
 `DIM` (arrays), `REM`, keyboard input, exponentiation (`A^B`), and cassette I/O.
 
 # Dependencies
diff --git a/exporter.h b/exporter.h
index bd03554..3a848bb 100644
--- a/exporter.h
+++ b/exporter.h
@@ -16,6 +16,7 @@ extern void tosgtax();
 extern void tosleax();
 extern void tosgeax();
 extern void bnegax();
+extern void negax();
 
 // Two bytes each.
 extern unsigned int ptr1;
diff --git a/exporter.s b/exporter.s
index e44c9a0..ee47161 100644
--- a/exporter.s
+++ b/exporter.s
@@ -31,6 +31,8 @@
 .export     _tosgeax := tosgeax
 .import     bnegax
 .export     _bnegax := bnegax
+.import     negax
+.export     _negax := negax
 
 .importzp   ptr1
 .exportzp   _ptr1 = ptr1
diff --git a/main.c b/main.c
index c2a5098..69e8485 100644
--- a/main.c
+++ b/main.c
@@ -516,6 +516,10 @@ static void pop_operator_stack() {
             add_call(bnegax);
             break;
 
+        case OP_NEG:
+            add_call(negax);
+            break;
+
         case OP_OPEN_PARENS:
             // No-op.
             break;
@@ -535,7 +539,7 @@ static void pop_operator_stack() {
  */
 static void push_operator_stack(uint8_t op) {
     // Don't pop anything off if op is unary.
-    if (op != OP_NOT) {
+    if (op != OP_NOT && op != OP_NEG) {
         // All our operators are left-associative, so no special check for the case
         // of equal precedence.
         while (g_op_stack_size > 0 &&
@@ -556,6 +560,7 @@ static void push_operator_stack(uint8_t op) {
  */
 static uint8_t *compile_expression(uint8_t *s) {
     char have_value_in_ax = 0;
+    uint8_t expect_unary_minus = 1; // Expect unary minus at start of expression.
 
     while (1) {
         if (IS_DIGIT(*s)) {
@@ -567,6 +572,9 @@ static uint8_t *compile_expression(uint8_t *s) {
 
             compile_load_ax(parse_uint16(&s));
             have_value_in_ax = 1;
+
+            // Expect binary minus after operand.
+            expect_unary_minus = 0;
         } else if (IS_FIRST_VARIABLE_LETTER(*s)) {
             // Variable reference.
             uint8_t var = find_variable(&s);
@@ -590,6 +598,9 @@ static uint8_t *compile_expression(uint8_t *s) {
                 g_c += 4;
             }
             have_value_in_ax = 1;
+
+            // Expect binary minus after operand.
+            expect_unary_minus = 0;
         } else {
             // Check if it's an operator.
             uint8_t op = OP_INVALID;
@@ -597,8 +608,7 @@ static uint8_t *compile_expression(uint8_t *s) {
             if (*s == T_PLUS) {
                 op = OP_ADD;
             } else if (*s == T_MINUS) {
-                // TODO check for unary.
-                op = OP_SUB;
+                op = expect_unary_minus ? OP_NEG : OP_SUB;
             } else if (*s == T_ASTERISK) {
                 op = OP_MULT;
             } else if (*s == T_SLASH) {
@@ -656,6 +666,10 @@ static uint8_t *compile_expression(uint8_t *s) {
                 }
             }
 
+            // Expect unary minus after operators or open parenthesis. Expect
+            // binary minus after close parenthesis.
+            expect_unary_minus = op != OP_CLOSE_PARENS;
+
             if (op != OP_INVALID) {
                 s += 1;
                 if (op != OP_CLOSE_PARENS) {