Why write this engine？
The reason is I coded a supercalculator last year in Appstore, which is based on one more completed parser.
This parser is just a simplified prototype. It calculates not only fraction but normal expression, so you can enrich this library in normal calculation. Detail illustration will be added in the near future.
Parser Process:
For example：1+2*（3-4） parsing process：
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Tokenizer: Tokens：1, +, 2, *, (, 3, -, 4, ）
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Interpreter： Specify the vague tokens，i.e '+' means add or positive ，the same as '-'.
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Parser
AST(抽象树)：
In fact, the math expression is parsed into one custom nested cluster in coding，and then resolve sub-expression on each level from inside to outside one.
Tips：
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adjust number, which is regarded as fraction through process，such as
2 convert to 2/1 ; 0.2 convert to 2/10; 0.9998 convert to 9998/10000
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because of object-c without function operator,we need to convert operators to function name, i.e @"+" to "add"
Todo ：processing more symbol