Allow REDEF for EQU constants

Fixes #853

src/asm/parser.y

@@ -851,6 +851,7 @@ directive	: endc
 		| warn
 		| assert
 		| def_equ
+		| redef_equ
 		| def_set
 		| def_rb
 		| def_rw
@@ -1118,6 +1119,11 @@ def_equ		: def_id T_POP_EQU const {
 		}
 ;
 
+redef_equ	: redef_id T_POP_EQU const {
+			sym_RedefEqu($1, $3);
+		}
+;
+
 def_set		: def_id set_or_equal const {
 			sym_AddSet($1, $3);
 		}

src/asm/rgbasm.5

@@ -967,6 +967,26 @@ def SCREEN_HEIGHT equ 144
 Note that colons
 .Ql \&:
 following the name are not allowed.
+.Pp
+If you
+.Em really
+need to, the
+.Ic REDEF
+keyword will define or redefine a constant symbol.
+This can be used, for example, to update a constant using a macro, without making it mutable in general.
+.Bd -literal -offset indent
+    def NUM_ITEMS equ 0
+MACRO add_item
+    redef NUM_ITEMS equ NUM_ITEMS + 1
+    def ITEM_{02x:NUM_ITEMS} equ \1
+ENDM
+    add_item 1
+    add_item 4
+    add_item 9
+    add_item 16
+    assert NUM_ITEMS == 4
+    assert ITEM_04 == 16
+.Ed
 .Ss Mutable constants
 .Ic SET ,
 or its synonym

src/asm/symbol.c

@@ -397,6 +397,30 @@ struct Symbol *sym_AddEqu(char const *symName, int32_t value)
 	return sym;
 }
 
+struct Symbol *sym_RedefEqu(char const *symName, int32_t value)
+{
+	struct Symbol *sym = sym_FindExactSymbol(symName);
+
+	if (!sym)
+		return sym_AddEqu(symName, value);
+
+	if (sym_IsDefined(sym) && sym->type != SYM_EQU) {
+		error("'%s' already defined as non-EQU at ", symName);
+		dumpFilename(sym);
+		putc('\n', stderr);
+		return NULL;
+	} else if (sym->isBuiltin) {
+		error("Built-in symbol '%s' cannot be redefined\n", symName);
+		return NULL;
+	}
+
+	updateSymbolFilename(sym);
+	sym->type = SYM_EQU;
+	sym->value = value;
+
+	return sym;
+}
+
 /*
  * Add a string equated symbol.
  *