/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */

use std::cmp::PartialEq;
use std::fmt;

use base::id::{BrowsingContextId, HistoryStateId, PipelineId, TopLevelBrowsingContextId};
use euclid::Size2D;
use log::debug;
use script_traits::LoadData;
use servo_url::ServoUrl;
use style_traits::CSSPixel;

use crate::browsingcontext::NewBrowsingContextInfo;

/// Represents the joint session history
/// <https://html.spec.whatwg.org/multipage/#joint-session-history>
#[derive(Debug)]
pub struct JointSessionHistory {
    /// Diffs used to traverse to past entries. Oldest entries are at the back,
    /// the most recent entries are at the front.
    pub past: Vec<SessionHistoryDiff>,

    /// Diffs used to traverse to future entries. Oldest entries are at the back,
    /// the most recent entries are at the front.
    pub future: Vec<SessionHistoryDiff>,
}

impl JointSessionHistory {
    pub fn new() -> JointSessionHistory {
        JointSessionHistory {
            past: Vec::new(),
            future: Vec::new(),
        }
    }

    pub fn history_length(&self) -> usize {
        self.past.len() + 1 + self.future.len()
    }

    pub fn push_diff(&mut self, diff: SessionHistoryDiff) -> Vec<SessionHistoryDiff> {
        debug!("pushing a past entry; removing future");
        self.past.push(diff);
        std::mem::take(&mut self.future)
    }

    pub fn replace_reloader(&mut self, old_reloader: NeedsToReload, new_reloader: NeedsToReload) {
        for diff in self.past.iter_mut().chain(self.future.iter_mut()) {
            diff.replace_reloader(&old_reloader, &new_reloader);
        }
    }

    pub fn replace_history_state(
        &mut self,
        pipeline_id: PipelineId,
        history_state_id: HistoryStateId,
        url: ServoUrl,
    ) {
        if let Some(SessionHistoryDiff::Pipeline {
            ref mut new_history_state_id,
            ref mut new_url,
            ..
        }) = self.past.iter_mut().find(|diff| match diff {
            SessionHistoryDiff::Pipeline {
                pipeline_reloader: NeedsToReload::No(id),
                ..
            } => pipeline_id == *id,
            _ => false,
        }) {
            *new_history_state_id = history_state_id;
            *new_url = url.clone();
        }

        if let Some(SessionHistoryDiff::Pipeline {
            ref mut old_history_state_id,
            ref mut old_url,
            ..
        }) = self.future.iter_mut().find(|diff| match diff {
            SessionHistoryDiff::Pipeline {
                pipeline_reloader: NeedsToReload::No(id),
                ..
            } => pipeline_id == *id,
            _ => false,
        }) {
            *old_history_state_id = Some(history_state_id);
            *old_url = url;
        }
    }

    pub fn remove_entries_for_browsing_context(&mut self, context_id: BrowsingContextId) {
        debug!("{}: Removing entries for browsing context", context_id);
        self.past.retain(|diff| match diff {
            SessionHistoryDiff::BrowsingContext {
                browsing_context_id,
                ..
            } => *browsing_context_id != context_id,
            _ => true,
        });
        self.future.retain(|diff| match diff {
            SessionHistoryDiff::BrowsingContext {
                browsing_context_id,
                ..
            } => *browsing_context_id != context_id,
            _ => true,
        });
    }
}

/// Represents a pending change in a session history, that will be applied
/// once the new pipeline has loaded and completed initial layout / paint.
#[derive(Debug)]
pub struct SessionHistoryChange {
    /// The browsing context to change.
    pub browsing_context_id: BrowsingContextId,

    /// The top-level browsing context ancestor.
    pub top_level_browsing_context_id: TopLevelBrowsingContextId,

    /// The pipeline for the document being loaded.
    pub new_pipeline_id: PipelineId,

    /// The old pipeline that the new pipeline should replace.
    pub replace: Option<NeedsToReload>,

    /// Holds data for not-yet constructed browsing contexts that are not
    /// easily available when they need to be constructed.
    pub new_browsing_context_info: Option<NewBrowsingContextInfo>,

    /// The size of the viewport for the browsing context.
    pub window_size: Size2D<f32, CSSPixel>,
}

/// Represents a pipeline or discarded pipeline in a history entry.
// FIXME: https://github.com/servo/servo/issues/34591
#[expect(clippy::large_enum_variant)]
#[derive(Clone, Debug)]
pub enum NeedsToReload {
    /// Represents a pipeline that has not been discarded
    No(PipelineId),
    /// Represents a pipeline that has been discarded and must be reloaded with the given `LoadData`
    /// if ever traversed to.
    Yes(PipelineId, LoadData),
}

impl fmt::Display for NeedsToReload {
    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            NeedsToReload::No(pipeline_id) => write!(fmt, "Alive({})", pipeline_id),
            NeedsToReload::Yes(pipeline_id, ..) => write!(fmt, "Dead({})", pipeline_id),
        }
    }
}

impl NeedsToReload {
    pub fn alive_pipeline_id(&self) -> Option<PipelineId> {
        match *self {
            NeedsToReload::No(pipeline_id) => Some(pipeline_id),
            NeedsToReload::Yes(..) => None,
        }
    }
}

// Custom `PartialEq` that only compares the `PipelineId`s of the same variants while ignoring `LoadData`
impl PartialEq for NeedsToReload {
    fn eq(&self, other: &NeedsToReload) -> bool {
        match *self {
            NeedsToReload::No(pipeline_id) => match *other {
                NeedsToReload::No(other_pipeline_id) => pipeline_id == other_pipeline_id,
                _ => false,
            },
            NeedsToReload::Yes(pipeline_id, _) => match *other {
                NeedsToReload::Yes(other_pipeline_id, _) => pipeline_id == other_pipeline_id,
                _ => false,
            },
        }
    }
}

/// Represents a the difference between two adjacent session history entries.
// FIXME: https://github.com/servo/servo/issues/34591
#[expect(clippy::large_enum_variant)]
#[derive(Debug)]
pub enum SessionHistoryDiff {
    /// Represents a diff where the active pipeline of an entry changed.
    BrowsingContext {
        /// The browsing context whose pipeline changed
        browsing_context_id: BrowsingContextId,
        /// The previous pipeline (used when traversing into the past)
        old_reloader: NeedsToReload,
        /// The next pipeline (used when traversing into the future)
        new_reloader: NeedsToReload,
    },
    /// Represents a diff where the active state of a pipeline changed.
    Pipeline {
        /// The pipeline whose history state changed.
        pipeline_reloader: NeedsToReload,
        /// The old history state id.
        old_history_state_id: Option<HistoryStateId>,
        /// The old url
        old_url: ServoUrl,
        /// The new history state id.
        new_history_state_id: HistoryStateId,
        /// The new url
        new_url: ServoUrl,
    },
    Hash {
        pipeline_reloader: NeedsToReload,
        old_url: ServoUrl,
        new_url: ServoUrl,
    },
}

impl SessionHistoryDiff {
    /// Returns the old pipeline id if that pipeline is still alive, otherwise returns `None`
    pub fn alive_old_pipeline(&self) -> Option<PipelineId> {
        match *self {
            SessionHistoryDiff::BrowsingContext {
                ref old_reloader, ..
            } => match *old_reloader {
                NeedsToReload::No(pipeline_id) => Some(pipeline_id),
                NeedsToReload::Yes(..) => None,
            },
            _ => None,
        }
    }

    /// Returns the new pipeline id if that pipeline is still alive, otherwise returns `None`
    pub fn alive_new_pipeline(&self) -> Option<PipelineId> {
        match *self {
            SessionHistoryDiff::BrowsingContext {
                ref new_reloader, ..
            } => match *new_reloader {
                NeedsToReload::No(pipeline_id) => Some(pipeline_id),
                NeedsToReload::Yes(..) => None,
            },
            _ => None,
        }
    }

    /// Replaces all occurances of the replaced pipeline with a new pipeline
    pub fn replace_reloader(
        &mut self,
        replaced_reloader: &NeedsToReload,
        reloader: &NeedsToReload,
    ) {
        match *self {
            SessionHistoryDiff::BrowsingContext {
                ref mut old_reloader,
                ref mut new_reloader,
                ..
            } => {
                if *old_reloader == *replaced_reloader {
                    *old_reloader = reloader.clone();
                }
                if *new_reloader == *replaced_reloader {
                    *new_reloader = reloader.clone();
                }
            },
            SessionHistoryDiff::Pipeline {
                ref mut pipeline_reloader,
                ..
            } => {
                if *pipeline_reloader == *replaced_reloader {
                    *pipeline_reloader = reloader.clone();
                }
            },
            SessionHistoryDiff::Hash {
                ref mut pipeline_reloader,
                ..
            } => {
                if *pipeline_reloader == *replaced_reloader {
                    *pipeline_reloader = reloader.clone();
                }
            },
        }
    }
}