//! RFC 6902 JSON Patch support.

use crate::PatchError;
use serde::{Deserialize, Serialize};
use serde_json::Value;

/// One RFC 6902 JSON Patch operation.
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
#[serde(tag = "op", rename_all = "lowercase")]
pub enum JsonPatchOperation {
    /// Add a value at the target JSON Pointer.
    Add {
        /// Target JSON Pointer.
        path: String,
        /// Value to insert.
        value: Value,
    },
    /// Remove the value at the target JSON Pointer.
    Remove {
        /// Target JSON Pointer.
        path: String,
    },
    /// Replace the value at the target JSON Pointer.
    Replace {
        /// Target JSON Pointer.
        path: String,
        /// Replacement value.
        value: Value,
    },
    /// Move a value from one JSON Pointer to another.
    Move {
        /// Source JSON Pointer.
        from: String,
        /// Target JSON Pointer.
        path: String,
    },
    /// Copy a value from one JSON Pointer to another.
    Copy {
        /// Source JSON Pointer.
        from: String,
        /// Target JSON Pointer.
        path: String,
    },
    /// Assert that a JSON Pointer currently contains a value.
    Test {
        /// Target JSON Pointer.
        path: String,
        /// Expected value.
        value: Value,
    },
}

/// Applies an RFC 6902 JSON Patch document to a JSON document string.
pub fn apply_json_patch(doc: &str, patch: &str) -> crate::Result<String> {
    let mut document: Value = serde_json::from_str(doc)
        .map_err(|error| PatchError::ApplyFailed(format!("invalid JSON document: {error}")))?;
    let operations: Vec<JsonPatchOperation> = serde_json::from_str(patch)
        .map_err(|error| PatchError::ApplyFailed(format!("invalid JSON patch: {error}")))?;
    apply_json_patch_value(&mut document, &operations)?;
    serde_json::to_string(&document)
        .map_err(|error| PatchError::ApplyFailed(format!("failed to serialize JSON: {error}")))
}

/// Applies parsed JSON Patch operations to a JSON value.
///
/// Each operation is applied atomically: when one operation fails, the document
/// remains at the state produced by the previous successful operation.
pub fn apply_json_patch_value(
    document: &mut Value,
    operations: &[JsonPatchOperation],
) -> crate::Result<()> {
    for operation in operations {
        let mut next = document.clone();
        apply_operation(&mut next, operation)?;
        *document = next;
    }
    Ok(())
}

fn apply_operation(document: &mut Value, operation: &JsonPatchOperation) -> crate::Result<()> {
    match operation {
        JsonPatchOperation::Add { path, value } => add_value(document, path, value.clone()),
        JsonPatchOperation::Remove { path } => remove_value(document, path).map(drop),
        JsonPatchOperation::Replace { path, value } => replace_value(document, path, value.clone()),
        JsonPatchOperation::Move { from, path } => {
            if from == path {
                return Ok(());
            }
            let value = get_value(document, from)?.clone();
            remove_value(document, from)?;
            add_value(document, path, value)
        }
        JsonPatchOperation::Copy { from, path } => {
            let value = get_value(document, from)?.clone();
            add_value(document, path, value)
        }
        JsonPatchOperation::Test { path, value } => {
            let actual = get_value(document, path)?;
            if actual == value {
                Ok(())
            } else {
                Err(failed(format!(
                    "JSON patch test failed at {path}: expected {value}, actual {actual}"
                )))
            }
        }
    }
}

fn add_value(document: &mut Value, path: &str, value: Value) -> crate::Result<()> {
    let tokens = parse_json_pointer(path)?;
    if tokens.is_empty() {
        *document = value;
        return Ok(());
    }

    let key = tokens.last().expect("checked non-empty").clone();
    let parent = get_mut_by_tokens(document, &tokens[..tokens.len() - 1])?;
    match parent {
        Value::Object(map) => {
            map.insert(key, value);
            Ok(())
        }
        Value::Array(items) => {
            if key == "-" {
                items.push(value);
                return Ok(());
            }
            let index = parse_array_index(&key, items.len(), true)?;
            items.insert(index, value);
            Ok(())
        }
        other => Err(failed(format!(
            "cannot add JSON patch value below non-container value {other}"
        ))),
    }
}

fn remove_value(document: &mut Value, path: &str) -> crate::Result<Value> {
    let tokens = parse_json_pointer(path)?;
    if tokens.is_empty() {
        return Ok(std::mem::take(document));
    }

    let key = tokens.last().expect("checked non-empty").clone();
    let parent = get_mut_by_tokens(document, &tokens[..tokens.len() - 1])?;
    match parent {
        Value::Object(map) => map
            .remove(&key)
            .ok_or_else(|| failed(format!("JSON patch path {path} does not exist"))),
        Value::Array(items) => {
            let index = parse_array_index(&key, items.len(), false)?;
            Ok(items.remove(index))
        }
        other => Err(failed(format!(
            "cannot remove JSON patch value below non-container value {other}"
        ))),
    }
}

fn replace_value(document: &mut Value, path: &str, value: Value) -> crate::Result<()> {
    let tokens = parse_json_pointer(path)?;
    if tokens.is_empty() {
        *document = value;
        return Ok(());
    }

    let key = tokens.last().expect("checked non-empty").clone();
    let parent = get_mut_by_tokens(document, &tokens[..tokens.len() - 1])?;
    match parent {
        Value::Object(map) => {
            let slot = map
                .get_mut(&key)
                .ok_or_else(|| failed(format!("JSON patch path {path} does not exist")))?;
            *slot = value;
            Ok(())
        }
        Value::Array(items) => {
            let index = parse_array_index(&key, items.len(), false)?;
            items[index] = value;
            Ok(())
        }
        other => Err(failed(format!(
            "cannot replace JSON patch value below non-container value {other}"
        ))),
    }
}

fn get_value<'a>(document: &'a Value, path: &str) -> crate::Result<&'a Value> {
    let tokens = parse_json_pointer(path)?;
    let mut current = document;
    for token in &tokens {
        current = match current {
            Value::Object(map) => map
                .get(token)
                .ok_or_else(|| failed(format!("JSON patch path {path} does not exist")))?,
            Value::Array(items) => {
                let index = parse_array_index(token, items.len(), false)?;
                items
                    .get(index)
                    .ok_or_else(|| failed(format!("JSON patch path {path} does not exist")))?
            }
            other => {
                return Err(failed(format!(
                    "cannot traverse JSON patch path {path} through non-container value {other}"
                )))
            }
        };
    }
    Ok(current)
}

fn get_mut_by_tokens<'a>(
    document: &'a mut Value,
    tokens: &[String],
) -> crate::Result<&'a mut Value> {
    let mut current = document;
    for token in tokens {
        current = match current {
            Value::Object(map) => map
                .get_mut(token)
                .ok_or_else(|| failed(format!("JSON patch path segment {token} does not exist")))?,
            Value::Array(items) => {
                let index = parse_array_index(token, items.len(), false)?;
                items.get_mut(index).ok_or_else(|| {
                    failed(format!("JSON patch path segment {token} does not exist"))
                })?
            }
            other => {
                return Err(failed(format!(
                    "cannot traverse JSON patch path through non-container value {other}"
                )))
            }
        };
    }
    Ok(current)
}

fn parse_array_index(token: &str, len: usize, allow_end: bool) -> crate::Result<usize> {
    if token.is_empty() || token == "-" {
        return Err(failed(format!("invalid JSON patch array index {token}")));
    }
    let index = token
        .parse::<usize>()
        .map_err(|_| failed(format!("invalid JSON patch array index {token}")))?;
    let max = if allow_end {
        len
    } else {
        len.checked_sub(1)
            .ok_or_else(|| failed("JSON patch array index exceeds empty array".to_string()))?
    };
    if index > max {
        return Err(failed(format!(
            "JSON patch array index {index} exceeds length {len}"
        )));
    }
    Ok(index)
}

fn parse_json_pointer(pointer: &str) -> crate::Result<Vec<String>> {
    if pointer.is_empty() {
        return Ok(Vec::new());
    }
    if !pointer.starts_with('/') {
        return Err(failed(format!(
            "JSON patch pointer must be empty or start with '/': {pointer}"
        )));
    }
    pointer[1..]
        .split('/')
        .map(decode_json_pointer_token)
        .collect()
}

fn decode_json_pointer_token(token: &str) -> crate::Result<String> {
    let mut decoded = String::with_capacity(token.len());
    let mut chars = token.chars();
    while let Some(character) = chars.next() {
        if character != '~' {
            decoded.push(character);
            continue;
        }
        match chars.next() {
            Some('0') => decoded.push('~'),
            Some('1') => decoded.push('/'),
            Some(other) => {
                return Err(failed(format!(
                    "invalid JSON patch pointer escape ~{other}"
                )))
            }
            None => return Err(failed("invalid JSON patch pointer escape ~".to_string())),
        }
    }
    Ok(decoded)
}

fn failed(message: String) -> PatchError {
    PatchError::ApplyFailed(message)
}

#[cfg(test)]
mod tests {
    use super::*;
    use serde_json::json;

    #[test]
    fn apply_json_patch_handles_all_core_operations() {
        let document = r#"{"name":"alice","items":["a","b"],"meta":{"keep":true}}"#;
        let patch = r#"[
            {"op":"test","path":"/meta/keep","value":true},
            {"op":"add","path":"/items/-","value":"c"},
            {"op":"replace","path":"/name","value":"bob"},
            {"op":"copy","from":"/meta","path":"/copied"},
            {"op":"move","from":"/items/0","path":"/first"},
            {"op":"remove","path":"/meta/keep"}
        ]"#;

        let output = apply_json_patch(document, patch).unwrap();
        let value: Value = serde_json::from_str(&output).unwrap();

        assert_eq!(value["name"], json!("bob"));
        assert_eq!(value["items"], json!(["b", "c"]));
        assert_eq!(value["first"], json!("a"));
        assert_eq!(value["copied"], json!({"keep": true}));
        assert_eq!(value["meta"], json!({}));
    }

    #[test]
    fn apply_json_patch_supports_pointer_escapes() {
        let document = r#"{"a/b":{"tilde~key":1}}"#;
        let patch = r#"[{"op":"replace","path":"/a~1b/tilde~0key","value":2}]"#;

        let output = apply_json_patch(document, patch).unwrap();
        let value: Value = serde_json::from_str(&output).unwrap();

        assert_eq!(value["a/b"]["tilde~key"], json!(2));
    }

    #[test]
    fn apply_json_patch_rejects_failed_test_without_mutating_value() {
        let mut value = json!({"enabled": true});
        let operations = vec![
            JsonPatchOperation::Add {
                path: "/count".to_string(),
                value: json!(1),
            },
            JsonPatchOperation::Test {
                path: "/enabled".to_string(),
                value: json!(false),
            },
        ];

        let error = apply_json_patch_value(&mut value, &operations).unwrap_err();

        assert!(matches!(error, crate::PatchError::ApplyFailed(_)));
        assert_eq!(value, json!({"enabled": true, "count": 1}));
    }

    #[test]
    fn apply_json_patch_rejects_missing_remove_path() {
        let error = apply_json_patch(r#"{"items":[]}"#, r#"[{"op":"remove","path":"/missing"}]"#)
            .unwrap_err();

        assert!(matches!(error, crate::PatchError::ApplyFailed(_)));
    }
}