use brk_computer::Computer;
use brk_error::{Error, Result};
use brk_types::{DifficultyAdjustmentEntry, Height};
use vecdb::{ReadableVec, Ro, VecIndex};

/// Walk every difficulty epoch overlapping `[start_height, end_height]` and
/// return one `DifficultyAdjustmentEntry` per retarget whose first block
/// lies inside the window. Each entry carries the epoch's first-block
/// timestamp/height, the epoch's difficulty, and the new/previous difficulty
/// ratio (e.g. 1.068 = +6.8%, matching the field's contract). Epochs whose
/// first block falls before `start_height` are skipped but their difficulty
/// is still read so the next in-window entry can compute its ratio. Returns
/// `Error::Internal` on any missing cursor read so corrupt zero-valued
/// entries cannot slip into the output under per-vec stamp lag.
pub(super) fn iter_difficulty_epochs(
    computer: &Computer<Ro>,
    start_height: usize,
    end_height: usize,
) -> Result<Vec<DifficultyAdjustmentEntry>> {
    let start_epoch = computer
        .indexes
        .height
        .epoch
        .collect_one(Height::from(start_height))
        .ok_or(Error::Internal(
            "iter_difficulty_epochs: start_height not in epoch index",
        ))?;
    let end_epoch = computer
        .indexes
        .height
        .epoch
        .collect_one(Height::from(end_height))
        .ok_or(Error::Internal(
            "iter_difficulty_epochs: end_height not in epoch index",
        ))?;

    let mut height_cursor = computer.indexes.epoch.first_height.cursor();
    let mut timestamp_cursor = computer.indexes.timestamp.epoch.cursor();
    let mut difficulty_cursor = computer.blocks.difficulty.value.epoch.cursor();

    let mut results = Vec::with_capacity(end_epoch.to_usize() - start_epoch.to_usize() + 1);
    let mut prev_difficulty: Option<f64> = None;

    for epoch_usize in start_epoch.to_usize()..=end_epoch.to_usize() {
        let epoch_height = height_cursor.get(epoch_usize).ok_or(Error::Internal(
            "iter_difficulty_epochs: missing epoch first_height",
        ))?;

        // Epochs that start before the window are skipped; we still record
        // their difficulty so the next in-window entry can compute its ratio.
        if epoch_height.to_usize() < start_height {
            prev_difficulty = Some(*difficulty_cursor.get(epoch_usize).ok_or(
                Error::Internal("iter_difficulty_epochs: missing pre-window epoch difficulty"),
            )?);
            continue;
        }

        let epoch_timestamp = timestamp_cursor.get(epoch_usize).ok_or(Error::Internal(
            "iter_difficulty_epochs: missing epoch timestamp",
        ))?;
        let epoch_difficulty = *difficulty_cursor.get(epoch_usize).ok_or(Error::Internal(
            "iter_difficulty_epochs: missing epoch difficulty",
        ))?;

        let change_percent = match prev_difficulty {
            Some(prev) if prev > 0.0 => epoch_difficulty / prev,
            _ => 0.0,
        };

        results.push(DifficultyAdjustmentEntry {
            timestamp: epoch_timestamp,
            height: epoch_height,
            difficulty: epoch_difficulty,
            change_percent,
        });

        prev_difficulty = Some(epoch_difficulty);
    }

    Ok(results)
}