public static ArrayList<Symbol> toSymbols(String expression) {
ArrayList<Symbol> symbols = new ArrayList<>();
for (int i = 0; i < expression.length(); i++) {
char c = expression.charAt(i);
if (isBracket(c)) {
// bracket
symbols.add(new Symbol(c));
} else if (c == ',') {
symbols.add(new Symbol(c));
} else if (isOperator(c)) {
if (c == '-') {
if (i == 0) {
// must be first +/-, so add 0
symbols.add(new Symbol(BigDecimal.valueOf(0)));
symbols.add(new Symbol(c));
} else {
char previous = expression.charAt(i - 1);
if ((previous == ')') || isNumber(String.valueOf(previous))) {
// must be "minus"
// e.g. sin(3)-5, 100-5,
symbols.add(new Symbol(c));
} else if ((previous == '(') || (previous == ',')) {
// must be "negative"
// e.g. (-3+5)/2
symbols.add(new Symbol(BigDecimal.valueOf(0)));
symbols.add(new Symbol(c));
} else if ((previous == '*') || (previous == '/') || (previous == '^')) {
int end = Symbol.getNumberEndAt(i + 1, expression);
symbols.add(new Symbol(new BigDecimal(expression.substring(i, end))));
i = end - 1;
}
}
} else {
symbols.add(new Symbol(c));
}
} else if (Character.isLetter(c)) {
// function
int end = Symbol.getFunctionEndAt(i, expression);
symbols.add(new Symbol(expression.substring(i, end)));
i = end - 1;
} else if (Character.isDigit(c)) {
int end = Symbol.getNumberEndAt(i, expression);
String number = expression.substring(i, end);
symbols.add(new Symbol(new BigDecimal(number)));
i = end - 1;
}
}
return symbols;
}
