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// A Simple, toy JSON parser.
// Remember there are Crates for This
extern crate scanlex;
use scanlex::{Scanner,Token,ScanError};
use std::collections::HashMap;
type JsonArray = Vec<Box<Value>>;
type JsonObject = HashMap<String,Box<Value>>;
#[derive(Debug, Clone, PartialEq)]
pub enum Value {
Str(String),
Num(f64),
Bool(bool),
Arr(JsonArray),
Obj(JsonObject),
Null
}
fn scan_json(scan: &mut Scanner) -> Result<Value,ScanError> {
use Value::*;
match scan.get() {
Token::Str(s) => Ok(Str(s)),
Token::Num(x) => Ok(Num(x)),
Token::Int(n) => Ok(Num(n as f64)),
Token::End => Err(scan.scan_error("unexpected end of input",None)),
Token::Error(e) => Err(e),
Token::Iden(s) =>
if s == "null" {Ok(Null)}
else if s == "true" {Ok(Bool(true))}
else if s == "false" {Ok(Bool(false))}
else {Err(scan.scan_error(&format!("unknown identifier '{}'",s),None))},
Token::Char(c) =>
if c == '[' {
let mut ja = Vec::new();
let mut ch = c;
while ch != ']' {
let o = scan_json(scan)?;
ch = scan.get_ch_matching(&[',',']'])?;
ja.push(Box::new(o));
}
Ok(Arr(ja))
} else
if c == '{' {
let mut jo = HashMap::new();
let mut ch = c;
while ch != '}' {
let key = scan.get_string()?;
scan.get_ch_matching(&[':'])?;
let o = scan_json(scan)?;
ch = scan.get_ch_matching(&[',','}'])?;
jo.insert(key,Box::new(o));
}
Ok(Obj(jo))
} else {
Err(scan.scan_error(&format!("bad char '{}'",c),None))
}
}
}
fn parse_json(txt: &str) -> Value {
let mut scan = Scanner::new(txt);
scan_json(&mut scan).expect("bad json")
}
use Value::*;
#[test]
fn array() {
let s = parse_json("[10,20]");
assert_eq!(s, Arr(vec![Box::new(Num(10.0)),Box::new(Num(20.0))]));
}
#[test]
fn array2() {
let s = parse_json("[null,true]");
assert_eq!(s, Arr(vec![Box::new(Null),Box::new(Bool(true))]));
}
#[test]
fn map() {
let s = parse_json("{'bonzo':10}");
let mut m = HashMap::new();
m.insert("bonzo".to_string(),Box::new(Num(10.0)));
assert_eq!(s, Obj(m));
}
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