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// =============================================================================
// Copyright (c) 2026 Haixing Hu.
//
// SPDX-License-Identifier: Apache-2.0
//
// Licensed under the Apache License, Version 2.0.
// =============================================================================
//! A guarded array or object frame used to copy one JSON tree depth-first.
use std::slice::Iter as SliceIter;
use serde_json::Map;
use serde_json::Value;
use serde_json::map::Iter as MapIter;
use super::SafeValueBuilder;
use crate::value::SafeJsonValue;
/// Owns one partially constructed container while its source children are
/// visited.
pub(crate) enum JsonValueCopyFrame<'a> {
/// An array frame keeps its source iterator and whether it owns a
/// builder-stack entry.
Array {
/// Remaining source array values.
values: SliceIter<'a, Value>,
/// Whether this frame has a builder for its remaining siblings.
has_builder: bool,
},
/// An object frame retains one key and whether it owns a builder-stack
/// entry.
Object {
/// Remaining source object entries.
values: MapIter<'a>,
/// Key associated with the currently visited child value.
pending_key: Option<&'a String>,
/// Whether this frame has a builder for its remaining siblings.
has_builder: bool,
},
}
impl<'a> JsonValueCopyFrame<'a> {
/// Creates an empty output frame over an array's borrowed entries.
///
/// # Parameters
///
/// * `values` - Source array entries borrowed for the frame's lifetime.
pub(crate) fn array(values: &'a [Value]) -> Self {
Self::Array {
values: values.iter(),
has_builder: false,
}
}
/// Creates an empty output frame over an object's borrowed entries.
///
/// # Parameters
///
/// * `values` - Source object entries borrowed for the frame's lifetime.
pub(crate) fn object(values: &'a Map<String, Value>) -> Self {
Self::Object {
values: values.iter(),
pending_key: None,
has_builder: false,
}
}
/// Returns the next child value, retaining its object key until insertion.
///
/// # Returns
///
/// The next borrowed child, or `None` when the source container is
/// exhausted. For object frames, its key remains pending until insertion.
pub(crate) fn next_value(&mut self) -> Option<&'a Value> {
match self {
Self::Array { values, .. } => values.next(),
Self::Object {
values, pending_key, ..
} => values.next().map(|(key, value)| {
*pending_key = Some(key);
value
}),
}
}
/// Reports whether source siblings remain after the child already fetched.
pub(crate) fn has_remaining(&self) -> bool {
match self {
Self::Array { values, .. } => values.len() > 0,
Self::Object { values, .. } => values.len() > 0,
}
}
/// Creates a guarded builder when this container has additional siblings.
///
/// # Parameters
///
/// * `builders` - Stack that receives this frame's partial container
/// builder.
pub(crate) fn start_builder(&mut self, builders: &mut Vec<SafeValueBuilder>) {
match self {
Self::Array { has_builder, .. } => {
builders.push(SafeValueBuilder::array());
*has_builder = true;
}
Self::Object { has_builder, .. } => {
builders.push(SafeValueBuilder::object());
*has_builder = true;
}
}
}
/// Adds a completed child, returning a finished owner for a single-child
/// container.
///
/// # Parameters
///
/// * `child` - Completed child value to insert into this container.
/// * `builders` - Active builder stack used when this frame owns a builder.
///
/// # Returns
///
/// `Some` with a finished container owner when this frame has no stack
/// builder; otherwise `None` after inserting the child into its builder.
pub(crate) fn push_child(
&mut self,
child: SafeJsonValue,
builders: &mut [SafeValueBuilder],
) -> Option<SafeJsonValue> {
match self {
Self::Array { has_builder: true, .. } => {
builders
.last_mut()
.expect("an active array frame has its builder")
.push(child);
None
}
Self::Array { has_builder: false, .. } => {
let mut builder = SafeValueBuilder::array();
builder.push(child);
Some(SafeJsonValue::new(builder.finish()))
}
Self::Object {
has_builder: true,
pending_key,
..
} => {
builders
.last_mut()
.expect("an active object frame has its builder")
.insert(
pending_key
.take()
.expect("an object child always has a pending key")
.clone(),
child,
);
None
}
Self::Object {
has_builder: false,
pending_key,
..
} => {
let mut builder = SafeValueBuilder::object();
builder.insert(
pending_key
.take()
.expect("an object child always has a pending key")
.clone(),
child,
);
Some(SafeJsonValue::new(builder.finish()))
}
}
}
/// Finishes an empty container without creating an active builder frame.
///
/// # Returns
///
/// An owner of the corresponding empty JSON array or object.
pub(crate) fn finish_empty(self) -> SafeJsonValue {
match self {
Self::Array { .. } => SafeJsonValue::new(SafeValueBuilder::array().finish()),
Self::Object { .. } => SafeJsonValue::new(SafeValueBuilder::object().finish()),
}
}
/// Finishes a container whose builder is the last active builder.
///
/// # Parameters
///
/// * `builders` - Stack whose final builder belongs to this frame.
///
/// # Returns
///
/// An owner of the completed JSON array or object.
pub(crate) fn finish(self, builders: &mut Vec<SafeValueBuilder>) -> SafeJsonValue {
match self {
Self::Array { has_builder: true, .. } | Self::Object { has_builder: true, .. } => {}
Self::Array { has_builder: false, .. } | Self::Object { has_builder: false, .. } => {
unreachable!("single-child frames finish when their child arrives")
}
}
let builder = builders.pop().expect("an active frame always has its builder");
SafeJsonValue::new(builder.finish())
}
}