//! Type-safe, bidirectional schema migration with compile-time chain validation.
//!
//! This module provides an Alembic-inspired migration system where every version
//! transition is a typed, bidirectional step between concrete Rust structs.
//!
//! # Architecture
//!
//! - [`MigrationStep`] — a single upgrade/downgrade between two concrete types
//! - [`MigrationChain`] — runtime dispatch generated by the `migration_chain!` macro
//! - `migration_chain!` — proc-macro (defined in `aimdb-derive`, re-exported here as
//!   `aimdb_data_contracts::migration_chain!`) that validates the chain at compile time
//!
//! # How It Works
//!
//! Each schema version is a concrete Rust struct. Migration steps convert between
//! adjacent versions with full type safety — no raw JSON manipulation.
//!
//! The `migration_chain!` macro generates, for a chain of any length:
//! 1. **Const assertions** — version sequence validated at compile time
//! 2. **Type-checked dispatch** — compiler rejects mismatched type chains
//! 3. **`MigrationChain` impl** — runtime upgrade/downgrade with version detection, `O(N)`
//!    in generated code size (one helper function per step, not one per historical version
//!    reachable from every other version)
//!
//! # Example
//!
//! ```rust
//! use aimdb_data_contracts::{SchemaType, MigrationStep, MigrationChain, MigrationError};
//! use aimdb_data_contracts::migration_chain;
//! use serde::{Deserialize, Serialize};
//!
//! // v1 schema
//! #[derive(Clone, Debug, Serialize, Deserialize)]
//! struct SensorV1 {
//!     schema_version: u32,
//!     temp: f32,
//!     timestamp: u64,
//! }
//! impl SchemaType for SensorV1 {
//!     const NAME: &'static str = "sensor_v1";
//!     const VERSION: u32 = 1;
//! }
//!
//! // v2 schema (current)
//! #[derive(Clone, Debug, Serialize, Deserialize)]
//! struct Sensor {
//!     schema_version: u32,
//!     celsius: f32,
//!     timestamp: u64,
//! }
//! impl SchemaType for Sensor {
//!     const NAME: &'static str = "sensor";
//!     const VERSION: u32 = 2;
//! }
//!
//! // Migration step: v1 -> v2
//! struct SensorV1ToV2;
//! impl MigrationStep for SensorV1ToV2 {
//!     type Older = SensorV1;
//!     type Newer = Sensor;
//!     const FROM_VERSION: u32 = 1;
//!     const TO_VERSION: u32 = 2;
//!
//!     fn up(v1: SensorV1) -> Result<Sensor, MigrationError> {
//!         Ok(Sensor { schema_version: 2, celsius: v1.temp, timestamp: v1.timestamp })
//!     }
//!     fn down(v2: Sensor) -> Result<SensorV1, MigrationError> {
//!         Ok(SensorV1 { schema_version: 1, temp: v2.celsius, timestamp: v2.timestamp })
//!     }
//! }
//!
//! // Wire up the chain
//! migration_chain! {
//!     type Current = Sensor;
//!     version_field = "schema_version";
//!     steps {
//!         SensorV1ToV2: SensorV1 => Sensor,
//!     }
//! }
//!
//! // Upgrade from v1 bytes
//! let v1_json = r#"{"schema_version":1,"temp":22.5,"timestamp":100}"#;
//! let sensor = Sensor::migrate_from_bytes(v1_json.as_bytes()).unwrap();
//! assert_eq!(sensor.celsius, 22.5);
//!
//! // Downgrade to v1 bytes
//! let v1_bytes = sensor.migrate_to_version(1).unwrap();
//! let v1_roundtrip: serde_json::Value = serde_json::from_slice(&v1_bytes).unwrap();
//! assert_eq!(v1_roundtrip["temp"], 22.5);
//! ```

use crate::SchemaType;

/// Error returned when schema migration fails.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum MigrationError {
    /// The source version is newer than this binary supports
    VersionTooNew { source: u32, current: u32 },
    /// The target downgrade version is below the minimum supported
    VersionTooOld { target: u32, minimum: u32 },
    /// Deserialization of a versioned payload failed
    DeserializationFailed(&'static str),
    /// Serialization during downgrade failed
    SerializationFailed(&'static str),
    /// A domain-specific conversion error in a MigrationStep
    ConversionFailed(&'static str),
    /// Payload is missing the version field
    MissingVersion,
}

impl core::fmt::Display for MigrationError {
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        match self {
            Self::VersionTooNew { source, current } => {
                write!(
                    f,
                    "source version {} is newer than current {}",
                    source, current
                )
            }
            Self::VersionTooOld { target, minimum } => {
                write!(
                    f,
                    "target version {} is below minimum supported {}",
                    target, minimum
                )
            }
            Self::DeserializationFailed(msg) => write!(f, "deserialization failed: {}", msg),
            Self::SerializationFailed(msg) => write!(f, "serialization failed: {}", msg),
            Self::ConversionFailed(msg) => write!(f, "conversion failed: {}", msg),
            Self::MissingVersion => write!(f, "payload missing version field"),
        }
    }
}

/// A single, typed, bidirectional migration step between two schema versions.
///
/// Each step converts between two concrete Rust types with full type safety.
/// The compiler enforces that `up()` and `down()` operate on the correct types.
///
/// # Example
///
/// ```rust
/// # use aimdb_data_contracts::{MigrationError, MigrationStep};
/// # struct TemperatureV1 { schema_version: u32, temp: f64, timestamp: u64, unit: String }
/// # struct TemperatureV2 { schema_version: u32, celsius: f64, timestamp: u64 }
/// struct TemperatureV1ToV2;
/// impl MigrationStep for TemperatureV1ToV2 {
///     type Older = TemperatureV1;
///     type Newer = TemperatureV2;
///     const FROM_VERSION: u32 = 1;
///     const TO_VERSION: u32 = 2;
///
///     fn up(v1: TemperatureV1) -> Result<TemperatureV2, MigrationError> {
///         let celsius = match v1.unit.as_str() {
///             "F" => (v1.temp - 32.0) * 5.0 / 9.0,
///             "K" => v1.temp - 273.15,
///             _ => v1.temp,
///         };
///         Ok(TemperatureV2 { schema_version: 2, celsius, timestamp: v1.timestamp })
///     }
///     fn down(v2: TemperatureV2) -> Result<TemperatureV1, MigrationError> {
///         Ok(TemperatureV1 { schema_version: 1, temp: v2.celsius, timestamp: v2.timestamp, unit: "C".into() })
///     }
/// }
/// ```
pub trait MigrationStep {
    /// The older schema type (input to `up`, output of `down`)
    type Older;
    /// The newer schema type (output of `up`, input to `down`)
    type Newer;
    /// The version number of the Older type
    const FROM_VERSION: u32;
    /// The version number of the Newer type
    const TO_VERSION: u32;

    /// Upgrade: convert from older to newer representation.
    fn up(older: Self::Older) -> Result<Self::Newer, MigrationError>;
    /// Downgrade: convert from newer to older representation.
    fn down(newer: Self::Newer) -> Result<Self::Older, MigrationError>;
}

/// A complete, validated migration chain for a schema type.
///
/// Generated by the `migration_chain!` macro. Provides runtime dispatch
/// for upgrading from any historical version to the current version,
/// and downgrading from the current version to any historical version.
///
/// All chain validation (sequential versions, type chaining) happens
/// at compile time via const assertions and type checking in the macro expansion.
pub trait MigrationChain: SchemaType + serde::de::DeserializeOwned + serde::Serialize {
    /// The minimum version this chain can upgrade from.
    const MIN_VERSION: u32;

    /// Deserialize from bytes, auto-detecting version and upgrading to current.
    ///
    /// Reads the version field from the JSON payload and walks the migration
    /// chain upward to produce the current schema version.
    fn migrate_from_bytes(data: &[u8]) -> Result<Self, MigrationError>;

    /// Downgrade to a target version and serialize to bytes.
    ///
    /// Walks the migration chain downward from the current version to produce
    /// the serialized representation of an older schema version.
    fn migrate_to_version(
        &self,
        target_version: u32,
    ) -> Result<alloc::vec::Vec<u8>, MigrationError>;
}

/// Compile-only proof that `migration_chain!` arity is unbounded — a 4-step
/// chain (5 schema versions), not itself a test. Runtime correctness for
/// chains beyond the historical 3-step ceiling is proven on host by
/// `tests/migration_roundtrip.rs`; this module exists purely so the
/// `thumbv7em-none-eabihf` check lane (which can't compile `tests/*.rs` —
/// no `std` test harness on a bare-metal target) still exercises a >3-step
/// chain. Lives inside this crate (not `tests/`), so it needs
/// `extern crate self as aimdb_data_contracts;` (see `lib.rs`) for the
/// macro's generated `::aimdb_data_contracts::...` absolute paths to
/// resolve.
#[allow(dead_code)]
mod arity_check {
    use crate::{MigrationError, MigrationStep, SchemaType};
    use serde::{Deserialize, Serialize};

    #[derive(Clone, Debug, Serialize, Deserialize)]
    struct V1 {
        schema_version: u32,
        n: u32,
    }
    impl SchemaType for V1 {
        const NAME: &'static str = "arity_check";
        const VERSION: u32 = 1;
    }

    #[derive(Clone, Debug, Serialize, Deserialize)]
    struct V2 {
        schema_version: u32,
        n: u32,
    }
    impl SchemaType for V2 {
        const NAME: &'static str = "arity_check";
        const VERSION: u32 = 2;
    }

    #[derive(Clone, Debug, Serialize, Deserialize)]
    struct V3 {
        schema_version: u32,
        n: u32,
    }
    impl SchemaType for V3 {
        const NAME: &'static str = "arity_check";
        const VERSION: u32 = 3;
    }

    #[derive(Clone, Debug, Serialize, Deserialize)]
    struct V4 {
        schema_version: u32,
        n: u32,
    }
    impl SchemaType for V4 {
        const NAME: &'static str = "arity_check";
        const VERSION: u32 = 4;
    }

    #[derive(Clone, Debug, Serialize, Deserialize)]
    struct V5 {
        schema_version: u32,
        n: u32,
    }
    impl SchemaType for V5 {
        const NAME: &'static str = "arity_check";
        const VERSION: u32 = 5;
    }

    struct Step1;
    impl MigrationStep for Step1 {
        type Older = V1;
        type Newer = V2;
        const FROM_VERSION: u32 = 1;
        const TO_VERSION: u32 = 2;
        fn up(v: V1) -> Result<V2, MigrationError> {
            Ok(V2 {
                schema_version: 2,
                n: v.n,
            })
        }
        fn down(v: V2) -> Result<V1, MigrationError> {
            Ok(V1 {
                schema_version: 1,
                n: v.n,
            })
        }
    }

    struct Step2;
    impl MigrationStep for Step2 {
        type Older = V2;
        type Newer = V3;
        const FROM_VERSION: u32 = 2;
        const TO_VERSION: u32 = 3;
        fn up(v: V2) -> Result<V3, MigrationError> {
            Ok(V3 {
                schema_version: 3,
                n: v.n,
            })
        }
        fn down(v: V3) -> Result<V2, MigrationError> {
            Ok(V2 {
                schema_version: 2,
                n: v.n,
            })
        }
    }

    struct Step3;
    impl MigrationStep for Step3 {
        type Older = V3;
        type Newer = V4;
        const FROM_VERSION: u32 = 3;
        const TO_VERSION: u32 = 4;
        fn up(v: V3) -> Result<V4, MigrationError> {
            Ok(V4 {
                schema_version: 4,
                n: v.n,
            })
        }
        fn down(v: V4) -> Result<V3, MigrationError> {
            Ok(V3 {
                schema_version: 3,
                n: v.n,
            })
        }
    }

    struct Step4;
    impl MigrationStep for Step4 {
        type Older = V4;
        type Newer = V5;
        const FROM_VERSION: u32 = 4;
        const TO_VERSION: u32 = 5;
        fn up(v: V4) -> Result<V5, MigrationError> {
            Ok(V5 {
                schema_version: 5,
                n: v.n,
            })
        }
        fn down(v: V5) -> Result<V4, MigrationError> {
            Ok(V4 {
                schema_version: 4,
                n: v.n,
            })
        }
    }

    crate::migration_chain! {
        type Current = V5;
        version_field = "schema_version";
        steps {
            Step1: V1 => V2,
            Step2: V2 => V3,
            Step3: V3 => V4,
            Step4: V4 => V5,
        }
    }
}