// =============================================================================
//    Copyright (c) 2026 Haixing Hu.
//
//    SPDX-License-Identifier: Apache-2.0
//
//    Licensed under the Apache License, Version 2.0.
// =============================================================================
//! Value-level convenience methods for low-level codecs.

use core::num::NonZeroUsize;

use crate::{
    CapacityError,
    Codec,
    CodecDecodeError,
    CodecEncodeError,
};

/// Extension trait for checked one-value codec operations.
///
/// `CodecValueExt` keeps convenience operations out of the low-level
/// [`Codec`] contract while still making them available to all codec
/// implementations. The methods compose primitive reset, encode, decode, and
/// flush hooks with capacity checks and adapter-level error wrapping.
pub trait CodecValueExt: Codec {
    /// Returns the maximum unit count emitted by one reset-prefixed value
    /// encode.
    ///
    /// This is the checked sum of
    /// [`Codec::max_encode_reset_units`] and
    /// [`Codec::max_units_per_value`]. It is useful for callers that want to
    /// reuse scratch storage for repeated one-value encodes.
    ///
    /// # Returns
    ///
    /// Returns the maximum reset-plus-value output length.
    ///
    /// # Errors
    ///
    /// Returns [`CapacityError::OutputLengthOverflow`] when the sum cannot be
    /// represented as `usize`.
    #[inline(always)]
    #[must_use = "capacity planning can fail on overflow"]
    fn max_encode_value_units(&self) -> Result<usize, CapacityError> {
        self.max_encode_reset_units()
            .checked_add(self.max_units_per_value().get())
            .ok_or(CapacityError::OutputLengthOverflow)
    }

    /// Encodes one value after emitting reset output into a caller buffer.
    ///
    /// The method validates the output index and the combined reset-plus-value
    /// capacity before calling the unchecked codec hooks. It is a convenience
    /// wrapper for code paths that need one complete value and want to reuse
    /// caller-owned storage.
    ///
    /// # Parameters
    ///
    /// - `value`: Value to encode.
    /// - `output`: Destination unit buffer.
    /// - `output_index`: Start index in `output`.
    ///
    /// # Returns
    ///
    /// Returns the total number of reset and value units written.
    ///
    /// # Errors
    ///
    /// Returns [`CodecEncodeError::UnencodableValue`] when `value` is outside
    /// this codec's encodable domain,
    /// [`CodecEncodeError::InvalidOutputIndex`] when `output_index` is outside
    /// `output`, [`CodecEncodeError::InsufficientOutput`] when the writable
    /// suffix cannot hold the reset output plus exact encoded value width,
    /// [`CodecEncodeError::OutputLengthOverflow`] when the bound overflows, or
    /// [`CodecEncodeError::Encode`] when reset or value encoding fails.
    ///
    /// # Panics
    ///
    /// Panics when the codec writes or reports more units than its declared
    /// reset or value bound.
    fn encode_value_with_reset(
        &mut self,
        value: &Self::Value,
        output: &mut [Self::Unit],
        output_index: usize,
    ) -> Result<usize, CodecEncodeError<Self::EncodeError>> {
        if !self.can_encode_value(value) {
            return Err(CodecEncodeError::unencodable_value(0));
        }
        let reset_units = self.max_encode_reset_units();
        let value_units = self.encode_len(value).get();
        let required = reset_units
            .checked_add(value_units)
            .ok_or_else(CodecEncodeError::output_length_overflow)?;
        CodecEncodeError::ensure_output_capacity(
            output.len(),
            output_index,
            required,
        )?;

        let reset_written = unsafe {
            // SAFETY: The capacity check above reserves the combined
            // reset-plus-value output bound at `output_index`.
            self.encode_reset(output, output_index)
        }
        .map_err(|error| CodecEncodeError::encode(error, 0))?;
        assert!(
            reset_written <= reset_units,
            "Codec::encode_reset wrote beyond its reset bound",
        );

        let value_written = unsafe {
            // SAFETY: `reset_written <= reset_units` and the earlier combined
            // capacity check leave the exact value width writable.
            self.encode(value, output, output_index + reset_written)
        }
        .map_err(|error| CodecEncodeError::encode(error, 0))?
        .get();
        assert!(
            value_written == value_units,
            "Codec::encode wrote a different length than Codec::encode_len",
        );
        Ok(reset_written + value_written)
    }

    /// Decodes one value and flushes decode-side state into caller storage.
    ///
    /// The method validates input and flush-output bounds before entering the
    /// unchecked codec hooks. It returns the decoded value, the consumed input
    /// count, and the number of flushed values written to `flush_output`.
    ///
    /// # Parameters
    ///
    /// - `input`: Source unit buffer.
    /// - `input_index`: Start index in `input`.
    /// - `flush_output`: Destination value buffer for decode-flush output.
    /// - `flush_output_index`: Start index in `flush_output`.
    ///
    /// # Returns
    ///
    /// Returns `(value, consumed, flushed)`.
    ///
    /// # Errors
    ///
    /// Returns [`CodecDecodeError::InvalidInputIndex`] when `input_index` is
    /// outside `input`, [`CodecDecodeError::Incomplete`] when fewer than
    /// [`Codec::min_units_per_value`] units are readable,
    /// [`CodecDecodeError::InvalidOutputIndex`] or
    /// [`CodecDecodeError::InsufficientOutput`] when flush output cannot hold
    /// [`Codec::max_decode_flush_values`], or [`CodecDecodeError::Decode`]
    /// when decoding or flushing fails.
    ///
    /// # Panics
    ///
    /// Panics when the codec consumes beyond available input or flushes more
    /// values than its declared bound.
    fn decode_value_with_flush(
        &mut self,
        input: &[Self::Unit],
        input_index: usize,
        flush_output: &mut [Self::Value],
        flush_output_index: usize,
    ) -> Result<
        (Self::Value, NonZeroUsize, usize),
        CodecDecodeError<Self::DecodeError>,
    > {
        CodecDecodeError::ensure_input_index(input.len(), input_index)?;
        let min_units = self.min_units_per_value().get();
        CodecDecodeError::ensure_min_input(
            input.len(),
            input_index,
            min_units,
        )?;

        let flush_cap = self.max_decode_flush_values();
        CodecDecodeError::ensure_output_capacity(
            flush_output.len(),
            flush_output_index,
            flush_cap,
        )?;

        let (value, consumed) = unsafe {
            // SAFETY: The input checks above guarantee the minimum readable
            // units required by `Codec::decode`.
            self.decode(input, input_index)
        }
        .map_err(|error| CodecDecodeError::decode(error, input_index))?;
        let available = input.len() - input_index;
        assert!(
            consumed.get() <= available,
            "Codec::decode consumed beyond available input",
        );

        let flushed = unsafe {
            // SAFETY: The flush-output checks above reserve the declared flush
            // output bound at `flush_output_index`.
            self.decode_flush(flush_output, flush_output_index)
        }
        .map_err(|error| {
            CodecDecodeError::decode(error, input_index + consumed.get())
        })?;
        assert!(
            flushed <= flush_cap,
            "Codec::decode_flush wrote beyond its flush bound",
        );
        Ok((value, consumed, flushed))
    }

    /// Decodes exactly one value and then flushes decode-side state.
    ///
    /// Unlike [`decode_value_with_flush`](Self::decode_value_with_flush), this
    /// helper requires the supplied input slice to contain exactly one encoded
    /// value. It validates trailing input before calling
    /// [`Codec::decode_flush`], preserving whole-value decoder semantics while
    /// still centralizing flush scratch-buffer handling.
    ///
    /// # Parameters
    ///
    /// - `input`: Source units for exactly one encoded value.
    /// - `flush_output`: Destination value buffer for decode-flush output.
    /// - `flush_output_index`: Start index in `flush_output`.
    ///
    /// # Returns
    ///
    /// Returns `(value, flushed)`.
    ///
    /// # Errors
    ///
    /// Returns [`CodecDecodeError::Incomplete`] when fewer than
    /// [`Codec::min_units_per_value`] units are available,
    /// [`CodecDecodeError::TrailingInput`] when decode succeeds but leaves
    /// extra units, output-capacity errors for invalid flush storage, or
    /// [`CodecDecodeError::Decode`] when decoding or flushing fails.
    ///
    /// # Panics
    ///
    /// Panics when the codec consumes beyond available input or flushes more
    /// values than its declared bound.
    fn decode_exact_value_with_flush(
        &mut self,
        input: &[Self::Unit],
        flush_output: &mut [Self::Value],
        flush_output_index: usize,
    ) -> Result<(Self::Value, usize), CodecDecodeError<Self::DecodeError>> {
        let min_units = self.min_units_per_value().get();
        CodecDecodeError::ensure_min_input(input.len(), 0, min_units)?;

        let flush_cap = self.max_decode_flush_values();
        CodecDecodeError::ensure_output_capacity(
            flush_output.len(),
            flush_output_index,
            flush_cap,
        )?;

        let (value, consumed) = unsafe {
            // SAFETY: The input check above guarantees the minimum readable
            // units required by `Codec::decode` at index 0.
            self.decode(input, 0)
        }
        .map_err(|error| CodecDecodeError::decode(error, 0))?;
        assert!(
            consumed.get() <= input.len(),
            "Codec::decode consumed beyond available input",
        );
        CodecDecodeError::ensure_no_trailing_input(
            consumed.get(),
            input.len(),
        )?;

        let flushed = unsafe {
            // SAFETY: The flush-output checks above reserve the declared flush
            // output bound at `flush_output_index`.
            self.decode_flush(flush_output, flush_output_index)
        }
        .map_err(|error| CodecDecodeError::decode(error, consumed.get()))?;
        assert!(
            flushed <= flush_cap,
            "Codec::decode_flush wrote beyond its flush bound",
        );
        Ok((value, flushed))
    }
}

impl<C> CodecValueExt for C where C: Codec + ?Sized {}
