*pragmapush list
                *pragma list
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; The LET command which is the default if no token begins a statement
cmd_let         jmp SNERROR                     ; not yet implemented
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; Expression Evaluation Package
;
; This is the expression evaluator. It handles everything from parsing numbers to dispatching function calls. The main
; entry point is eval_expr which will evaluate an arbitrary expression. It returns as soon as it reaches something it
; doesn't understand as part of an expression.
;
; The special handling for relational operators is required because Basic allows them in all 
eval_expr       clrb                            ; flag previous operator as minimum precdence (end of expression)
eval_expraux    jsr eval_term                   ; evaluate the first term of the expression
eval_expr0      jsr curchar                     ; fetch current input
                beq eval_expr1                  ; brif end of expression - we're done
                cmpa #tok_or                    ; is it above operators?
                bhi eval_expr1                  ; brif so
                suba #tok_plus                  ; offset to zero for first operator token
                bcc eval_expr2                  ; brif it is an operator
eval_expr1      rts
eval_expr2      pshs b                          ; save previous operator precedence
                ldx #oper_tab                   ; point to operator table
                tfr a,b                         ; shift to B for "ABX"
                abx                             ; add three times (3 bytes per entry)
                abx                             ; OBS: TFR + ABX + ABX + ABX is faster than LDB + MUL + ABX
                abx                             ; now X points to the operator entry in the table
                ldb ,x                          ; get precedence of current operation
                cmpb ,s                         ; is it higher than the current operation?
                bhi eval_expr3                  ; brif so - process this operator
                puls b,pc                       ; return current value to complete previous operation
eval_expr3      jsr nextchar                    ; eat the operator token
                ldx 1,x                         ; get handler address of this operator
                leas -val.size,s                ; make room for the result accumulator
                pshs x                          ; save handler address for later
                lda val0+val.type               ; get current value type
                ldx val0                        ; get value accumlator contents (6 bytes)
                ldy val0+2
                ldu val0+4
                pshs a,x,y,u                    ; save it on the stack
                jsr eval_expraux                ; evaluate the following term and higher precedence expressions
                puls a,x,y,u                    ; get back saved value
                stx val1                        ; save it to the second value accumulator
                sty val1+2
                stu val1+4
                sta val1+val.type               ; save previous value type
                ldx #val1                       ; point to left operand
                ldu #val0                       ; point to right operand
                leay 2,s                        ; point to return value location
                jsr [,s++]                      ; go handle the operator
                puls a,x,y,u                    ; get return value
                sta val0
                stx val0+1
                sty val0+3
                stu val0+5
                puls b                          ; get back the previous operator precedence
                bra eval_expr0                  ; go process another operator or end of expression
eval_term       jsr curchar                     ; get current input character
                beq eval_term0                  ; brif end of input - this is an error
                bcs eval_number                 ; brif digit - we have a number
;                bmi eval_func                   ; brif we have a token - handle function call
                cmpa #'.                        ; decimal point?
                beq eval_number                 ; brif so - evaluate number
                cmpa #'-                        ; negative sign?
                beq eval_number                 ; brif so - evaluate number
                cmpa #'+                        ; positive sign?
                beq eval_number                 ; brif so - evaluate number
eval_term0      jmp SNERROR                     ; we have something unrecognized - raise error
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; Evaluate a numeric constant. This process works as follows:
;
; 0. Clear the value to a zero integer
; 1. Check for signs and flag appropriately
; 2. Read character
; 3. If decimal or exponential indicator, go to step 6
; 4. If not digit, return integer result
; 5. Multiply accumulator by 10 and add digit value; go back to step 2
; 6. Convert accumulator to floating point; set accumulated decimal exponent and decimal flag to 0
; 7. If decimal point, flag decimal seen (0xff) and go to step 15 (or raise error if second decimal point)
; 8. If digit, multiply by 10 and add digit value; go to step 15
; 9. If E or e, go handle decimal exponent at step 12
; 10. Apply accumulated decimal exponent to the result (through multiplication/division by 10)
; 11. Return floating point result
; 12. Read character
; 13. If not digit, go handle return at step 10
; 14. Multiply exponent accumulator by 10 and add digit value; raise error on overflow or go back to step 12
; 15. Read a character and go to step 7
;
; If the result ends up being larger than a floating point value can hold, return Overflow
eval_number     jmp val_parsenum                ; if we don't recognize anything else, just parse a numer
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; Operator table
;
; Each entry starts with the precedence value followed by the handler routine. Each handler will receive its left
; operand in val1 and its right operand in val0 and should return its result in val0.
oper_tab        fcb 0x79                        ; addition
                fdb oper_plus
                fcb 0x79                        ; subtraction
                fdb oper_minus
                fcb 0x7b                        ; multiplication
                fdb SNERROR
                fcb 0x7b                        ; division
                fdb SNERROR
                fcb 0x7f                        ; exponentiation
                fdb SNERROR
                fcb 0x64                        ; less than
                fdb SNERROR
                fcb 0x64                        ; equal to
                fdb SNERROR
                fcb 0x64                        ; greater than
                fdb SNERROR
                fcb 0x64                        ; less than or equal to
                fdb SNERROR
                fcb 0x64                        ; greater than or equal to
                fdb SNERROR
                fcb 0x64                        ; not equal to
                fdb SNERROR
                fcb 0x50                        ; boolean AND
                fdb SNERROR
                fcb 0x46                        ; boolean OR
                fdb SNERROR
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; Operator handling routines
;
; binary plus: addition and concatenation
oper_plus       ldb val.type,x                  ; get type of the left operand
                cmpb valtype_string             ; is it string?
                bne oper_plus0                  ; brif not
                cmpb val.type,u                 ; is right operand also string?
                lbeq SNERROR                    ; brif so - do string concatenation
oper_plus0      jsr val_matchtypes              ; go match data types
                jmp val_add                     ; go add the values
; binary minus: subtraction
oper_minus      jsr val_matchtypes              ; go match data types
                jmp val_sub                     ; do subtraction
                *pragmapop list