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import { getAssociativity, getPrecedence, getArgsLen } from "./helpers";
import {
Associativity,
RPNToken,
Token,
isRPNToken,
getOperatorArgsLen,
} from "./types";
const shuntingYard = (input: Token[]): RPNToken[] => {
if (input.length === 0) {
// Input was empty
return [{ type: "VAL", value: 0 }];
}
const output: RPNToken[] = [];
const operatorStack: Token[] = [];
const functionStack: Token[] = [];
let prevToken: Token | undefined;
for (const token of input) {
// If the current Token is a value or a variable, put them into the output stream.
if (
token.type === "VAL" ||
token.type === "VAR" ||
token.type === "CONST"
) {
output.push(token);
prevToken = token;
continue;
}
// If the current Token is a function, put it onto the operator stack.
if (token.type === "FUN") {
operatorStack.push(token);
functionStack.push(token);
prevToken = token;
continue;
}
/*
* If the current Token is a function argument separator, pop operators
* to output stream until a left brace is encountered.
*/
if (token.type === "SEPERATOR") {
while (
operatorStack.length > 0 &&
operatorStack[operatorStack.length - 1].type !== "LBRACE"
) {
const stackTail = operatorStack.pop();
Iif (stackTail && isRPNToken(stackTail)) {
output.push(stackTail);
}
}
// If no left brace is encountered, separator was misplaced or parenthesis mismatch
Iif (
operatorStack.length > 0 &&
operatorStack[operatorStack.length - 1].type !== "LBRACE"
) {
// TODO never reached, check this.
throw new Error("Misplaced separator or mismatched parenthesis.");
}
functionStack.push(token);
prevToken = token;
continue;
}
// /* if the current Tokens type is MINUS and the previous Token is an operator or type LBRACE
// * or we're at the beginning of the expression (prevToken == null) the current Token is
// * an unary minus, so the tokentype has to be changed.
// */
// if (
// token.type === "OP" &&
// token.operator === "-" &&
// (prevToken === undefined ||
// prevToken.type === "OP" ||
// prevToken.type === "LBRACE")
// ) {
// const newToken: Token = {
// type: "OP",
// operator: "u",
// };
// operatorStack.push(newToken);
// prevToken = newToken;
// continue;
// }
/*
* If the current token is an operator and it's priority is lower than the priority of the last
* operator in the operator buffer, than put the operators from the operator buffer into the output
* stream until you find an operator with a priority lower or equal as the current tokens.
* Then add the current Token to the operator buffer.
*/
if (token.type === "OP") {
while (
operatorStack.length > 0 &&
// Left Associative
((getAssociativity(token) === Associativity.Left &&
getPrecedence(token) <=
getPrecedence(operatorStack[operatorStack.length - 1])) ||
// Right Associative
(getAssociativity(token) === Associativity.Right &&
getPrecedence(token) <
getPrecedence(operatorStack[operatorStack.length - 1])))
) {
const stackTail = operatorStack.pop();
if (stackTail && isRPNToken(stackTail)) {
output.push(stackTail);
}
}
operatorStack.push(token);
prevToken = token;
continue;
}
// If the current Token is a left brace, put it on the operator buffer.
if (token.type === "LBRACE") {
if (prevToken?.type === "VAR") {
throw new Error(`${prevToken.symbol}() is not a function`);
}
if (prevToken?.type === "VAL") {
throw new Error(`${prevToken.value}() is not a function`);
}
Iif (prevToken?.type === "CONST") {
throw new Error(`Const ${prevToken.symbol}() is not a function`);
}
operatorStack.push(token);
prevToken = token;
continue;
}
// If the current Token is a right brace, empty the operator buffer until you find a left brace.
if (token.type === "RBRACE") {
while (
operatorStack.length > 0 &&
operatorStack[operatorStack.length - 1].type !== "LBRACE"
) {
const stackTail = operatorStack.pop();
if (stackTail && isRPNToken(stackTail)) {
output.push(stackTail);
}
}
// Expect next token on stack to be left parenthesis and pop it
if (
operatorStack.length === 0 ||
operatorStack.pop()?.type !== "LBRACE"
) {
throw new Error("Mismatched parenthesis.");
}
// If the token at the top of the stack is a function token, pop it onto the output queue.
if (
operatorStack.length > 0 &&
operatorStack[operatorStack.length - 1].type === "FUN"
) {
const stackTail = operatorStack.pop();
if (stackTail && isRPNToken(stackTail)) {
output.push(stackTail);
}
// Check number of args is correct
let numArgs = 1;
while (
functionStack.length > 0 &&
functionStack[functionStack.length - 1].type !== "FUN"
) {
functionStack.pop();
numArgs++;
}
Iif (
stackTail?.type === "FUN" &&
numArgs !== getArgsLen(stackTail.function)
) {
throw new Error(
`Unexpected number of args passed to function "${
stackTail.function
}". Found ${numArgs} but expected ${getArgsLen(stackTail.function)}`
);
}
functionStack.pop();
}
}
prevToken = token;
}
/*
* When the algorithm reaches the end of the input stream, we add the
* tokens in the operatorBuffer to the outputStream. If the operator
* on top of the stack is a parenthesis, there are mismatched parenthesis.
*/
while (operatorStack.length > 0) {
if (
operatorStack[operatorStack.length - 1].type === "LBRACE" ||
operatorStack[operatorStack.length - 1].type === "RBRACE"
) {
throw new Error("Mismatched parenthesis.");
}
const stackTail = operatorStack.pop();
if (stackTail && isRPNToken(stackTail)) {
output.push(stackTail);
}
}
// Validate output
let stackCount = 0;
let lastOp = "";
for (const token of output) {
if (
token.type === "VAL" ||
token.type === "VAR" ||
token.type === "CONST"
) {
stackCount++;
} else if (token.type === "FUN") {
lastOp = token.function;
stackCount -= getArgsLen(token.function) - 1;
} else if (token.type === "OP") {
lastOp = token.operator;
stackCount -= getOperatorArgsLen(token.operator) - 1;
} else E{
/* istanbul ignore next */
assertUnreachable(token);
}
if (stackCount <= 0) {
throw new Error(`Not enough operands for ${lastOp}.`);
}
}
if (stackCount > 1) {
throw new Error(`Invalid expression.`);
}
return output;
};
export default shuntingYard;
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