use super::Value;
use crate::map::Map;
use alloc::borrow::ToOwned;
use alloc::string::String;
use core::fmt::{self, Display};
use core::ops;
/// A type that can be used to index into a `serde_json::Value`.
///
/// The [`get`] and [`get_mut`] methods of `Value` accept any type that
/// implements `Index`, as does the [square-bracket indexing operator]. This
/// trait is implemented for strings which are used as the index into a JSON
/// map, and for `usize` which is used as the index into a JSON array.
///
/// [`get`]: ../enum.Value.html#method.get
/// [`get_mut`]: ../enum.Value.html#method.get_mut
/// [square-bracket indexing operator]: ../enum.Value.html#impl-Index%3CI%3E-for-Value
///
/// This trait is sealed and cannot be implemented for types outside of
/// `serde_json`.
///
/// # Examples
///
/// ```
/// # use serde_json::json;
/// #
/// let data = json!({ "inner": [1, 2, 3] });
///
/// // Data is a JSON map so it can be indexed with a string.
/// let inner = &data["inner"];
///
/// // Inner is a JSON array so it can be indexed with an integer.
/// let first = &inner[0];
///
/// assert_eq!(first, 1);
/// ```
pub trait Index: private::Sealed {
/// Return None if the key is not already in the array or object.
#[doc(hidden)]
fn index_into<'v>(&self, v: &'v Value) -> Option<&'v Value>;
/// Return None if the key is not already in the array or object.
#[doc(hidden)]
fn index_into_mut<'v>(&self, v: &'v mut Value) -> Option<&'v mut Value>;
/// Panic if array index out of bounds. If key is not already in the object,
/// insert it with a value of null. Panic if Value is a type that cannot be
/// indexed into, except if Value is null then it can be treated as an empty
/// object.
#[doc(hidden)]
fn index_or_insert<'v>(&self, v: &'v mut Value) -> &'v mut Value;
}
impl Index for usize {
fn index_into<'v>(&self, v: &'v Value) -> Option<&'v Value> {
match v {
Value::Array(vec) => vec.get(*self),
_ => None,
}
}
fn index_into_mut<'v>(&self, v: &'v mut Value) -> Option<&'v mut Value> {
match v {
Value::Array(vec) => vec.get_mut(*self),
_ => None,
}
}
fn index_or_insert<'v>(&self, v: &'v mut Value) -> &'v mut Value {
match v {
Value::Array(vec) => {
let len = vec.len();
vec.get_mut(*self).unwrap_or_else(|| {
panic!(
"cannot access index {} of JSON array of length {}",
self, len
)
})
}
_ => panic!("cannot access index {} of JSON {}", self, Type(v)),
}
}
}
impl Index for str {
fn index_into<'v>(&self, v: &'v Value) -> Option<&'v Value> {
match v {
Value::Object(map) => map.get(self),
_ => None,
}
}
fn index_into_mut<'v>(&self, v: &'v mut Value) -> Option<&'v mut Value> {
match v {
Value::Object(map) => map.get_mut(self),
_ => None,
}
}
fn index_or_insert<'v>(&self, v: &'v mut Value) -> &'v mut Value {
if let Value::Null = v {
*v = Value::Object(Map::new());
}
match v {
Value::Object(map) => map.entry(self.to_owned()).or_insert(Value::Null),
_ => panic!("cannot access key {:?} in JSON {}", self, Type(v)),
}
}
}
impl Index for String {
fn index_into<'v>(&self, v: &'v Value) -> Option<&'v Value> {
self[..].index_into(v)
}
fn index_into_mut<'v>(&self, v: &'v mut Value) -> Option<&'v mut Value> {
self[..].index_into_mut(v)
}
fn index_or_insert<'v>(&self, v: &'v mut Value) -> &'v mut Value {
self[..].index_or_insert(v)
}
}
impl<T> Index for &T
where
T: ?Sized + Index,
{
fn index_into<'v>(&self, v: &'v Value) -> Option<&'v Value> {
(**self).index_into(v)
}
fn index_into_mut<'v>(&self, v: &'v mut Value) -> Option<&'v mut Value> {
(**self).index_into_mut(v)
}
fn index_or_insert<'v>(&self, v: &'v mut Value) -> &'v mut Value {
(**self).index_or_insert(v)
}
}
// Prevent users from implementing the Index trait.
mod private {
pub trait Sealed {}
impl Sealed for usize {}
impl Sealed for str {}
impl Sealed for alloc::string::String {}
impl<'a, T> Sealed for &'a T where T: ?Sized + Sealed {}
}
/// Used in panic messages.
struct Type<'a>(&'a Value);
impl<'a> Display for Type<'a> {
fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
match *self.0 {
Value::Null => formatter.write_str("null"),
Value::Bool(_) => formatter.write_str("boolean"),
Value::Number(_) => formatter.write_str("number"),
Value::String(_) => formatter.write_str("string"),
Value::Array(_) => formatter.write_str("array"),
Value::Object(_) => formatter.write_str("object"),
}
}
}
// The usual semantics of Index is to panic on invalid indexing.
//
// That said, the usual semantics are for things like Vec and BTreeMap which
// have different use cases than Value. If you are working with a Vec, you know
// that you are working with a Vec and you can get the len of the Vec and make
// sure your indices are within bounds. The Value use cases are more
// loosey-goosey. You got some JSON from an endpoint and you want to pull values
// out of it. Outside of this Index impl, you already have the option of using
// value.as_array() and working with the Vec directly, or matching on
// Value::Array and getting the Vec directly. The Index impl means you can skip
// that and index directly into the thing using a concise syntax. You don't have
// to check the type, you don't have to check the len, it is all about what you
// expect the Value to look like.
//
// Basically the use cases that would be well served by panicking here are
// better served by using one of the other approaches: get and get_mut,
// as_array, or match. The value of this impl is that it adds a way of working
// with Value that is not well served by the existing approaches: concise and
// careless and sometimes that is exactly what you want.
impl<I> ops::Index<I> for Value
where
I: Index,
{
type Output = Value;
/// Index into a `serde_json::Value` using the syntax `value[0]` or
/// `value["k"]`.
///
/// Returns `Value::Null` if the type of `self` does not match the type of
/// the index, for example if the index is a string and `self` is an array
/// or a number. Also returns `Value::Null` if the given key does not exist
/// in the map or the given index is not within the bounds of the array.
///
/// For retrieving deeply nested values, you should have a look at the
/// `Value::pointer` method.
///
/// # Examples
///
/// ```
/// # use serde_json::json;
/// #
/// let data = json!({
/// "x": {
/// "y": ["z", "zz"]
/// }
/// });
///
/// assert_eq!(data["x"]["y"], json!(["z", "zz"]));
/// assert_eq!(data["x"]["y"][0], json!("z"));
///
/// assert_eq!(data["a"], json!(null)); // returns null for undefined values
/// assert_eq!(data["a"]["b"], json!(null)); // does not panic
/// ```
fn index(&self, index: I) -> &Value {
static NULL: Value = Value::Null;
index.index_into(self).unwrap_or(&NULL)
}
}
impl<I> ops::IndexMut<I> for Value
where
I: Index,
{
/// Write into a `serde_json::Value` using the syntax `value[0] = ...` or
/// `value["k"] = ...`.
///
/// If the index is a number, the value must be an array of length bigger
/// than the index. Indexing into a value that is not an array or an array
/// that is too small will panic.
///
/// If the index is a string, the value must be an object or null which is
/// treated like an empty object. If the key is not already present in the
/// object, it will be inserted with a value of null. Indexing into a value
/// that is neither an object nor null will panic.
///
/// # Examples
///
/// ```
/// # use serde_json::json;
/// #
/// let mut data = json!({ "x": 0 });
///
/// // replace an existing key
/// data["x"] = json!(1);
///
/// // insert a new key
/// data["y"] = json!([false, false, false]);
///
/// // replace an array value
/// data["y"][0] = json!(true);
///
/// // inserted a deeply nested key
/// data["a"]["b"]["c"]["d"] = json!(true);
///
/// println!("{}", data);
/// ```
fn index_mut(&mut self, index: I) -> &mut Value {
index.index_or_insert(self)
}
}
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