use std::fmt;

use super::{EntityId, EntityKind};

/// Top-level error type. Every public API method returns `Result<T, LumenError>`.
#[derive(Debug)]
pub enum LumenError {
    /// Storage engine failure. Retry may succeed.
    Storage(StorageError),
    /// Entity not found. Caller should handle.
    NotFound { kind: EntityKind, id: EntityId },
    /// Schema violation. Caller's fault — fix the input.
    Schema(SchemaError),
    /// Signal write failed durability check. Retry required.
    Durability(DurabilityError),
    /// Query malformed. Parse error with details.
    Query(QueryError),
    /// Internal invariant violated. This is a bug in Lumen.
    Internal(String),
}

impl fmt::Display for LumenError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::Storage(e) => write!(f, "storage error: {e}"),
            Self::NotFound { kind, id } => write!(f, "{kind} {id} not found"),
            Self::Schema(e) => write!(f, "{e}"),
            Self::Durability(e) => write!(f, "durability error: {e}"),
            Self::Query(e) => write!(f, "query error: {e}"),
            Self::Internal(msg) => write!(f, "internal error: {msg}"),
        }
    }
}

impl std::error::Error for LumenError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        match self {
            Self::Storage(e) => Some(e),
            Self::Schema(e) => Some(e),
            Self::Durability(e) => Some(e),
            Self::Query(e) => Some(e),
            Self::NotFound { .. } | Self::Internal(_) => None,
        }
    }
}

impl From<SchemaError> for LumenError {
    fn from(e: SchemaError) -> Self {
        Self::Schema(e)
    }
}

impl From<StorageError> for LumenError {
    fn from(e: StorageError) -> Self {
        Self::Storage(e)
    }
}

impl From<DurabilityError> for LumenError {
    fn from(e: DurabilityError) -> Self {
        Self::Durability(e)
    }
}

impl From<QueryError> for LumenError {
    fn from(e: QueryError) -> Self {
        Self::Query(e)
    }
}

/// Schema validation errors.
///
/// `Eq` is manually implemented because f64 fields (from `Duration::as_secs_f64()`)
/// are always non-NaN, making equality reflexive.
#[derive(Debug, Clone, PartialEq)]
pub enum SchemaError {
    DuplicateSignalName(String),
    InvalidSignalName(String),
    InvalidHalfLife {
        signal_name: String,
        half_life_secs: f64,
    },
    InvalidLifetime {
        signal_name: String,
        lifetime_secs: f64,
    },
    EmptyWindows {
        signal_name: String,
    },
    VelocityWithoutWindows {
        signal_name: String,
    },
    NoSignalsDefined,
}

impl Eq for SchemaError {}

impl fmt::Display for SchemaError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::DuplicateSignalName(name) => {
                write!(f, "duplicate signal name: '{name}'")
            }
            Self::InvalidSignalName(name) => {
                write!(f, "invalid signal name: '{name}'")
            }
            Self::InvalidHalfLife {
                signal_name,
                half_life_secs,
            } => {
                write!(
                    f,
                    "signal '{signal_name}': invalid half-life: {half_life_secs}s"
                )
            }
            Self::InvalidLifetime {
                signal_name,
                lifetime_secs,
            } => {
                write!(
                    f,
                    "signal '{signal_name}': invalid lifetime: {lifetime_secs}s"
                )
            }
            Self::EmptyWindows { signal_name } => {
                write!(
                    f,
                    "signal '{signal_name}': non-permanent signal requires at least one window"
                )
            }
            Self::VelocityWithoutWindows { signal_name } => {
                write!(
                    f,
                    "signal '{signal_name}': velocity requires at least one window"
                )
            }
            Self::NoSignalsDefined => f.write_str("schema must define at least one signal"),
        }
    }
}

impl std::error::Error for SchemaError {}

/// Re-exported from `crate::storage::StorageError`.
pub use crate::storage::StorageError;

/// Stub for Phase 1.2+.
#[derive(Debug)]
pub struct DurabilityError {
    pub message: String,
}

impl fmt::Display for DurabilityError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(&self.message)
    }
}

impl std::error::Error for DurabilityError {}

/// Stub for Milestone 2+.
#[derive(Debug)]
pub struct QueryError {
    pub message: String,
}

impl fmt::Display for QueryError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str(&self.message)
    }
}

impl std::error::Error for QueryError {}

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

    #[test]
    fn lumen_error_display_not_found() {
        let e = LumenError::NotFound {
            kind: EntityKind::Item,
            id: EntityId::new(42),
        };
        assert_eq!(e.to_string(), "item 42 not found");
    }

    #[test]
    fn lumen_error_display_schema() {
        let e = LumenError::Schema(SchemaError::DuplicateSignalName("view".into()));
        assert!(e.to_string().contains("duplicate signal name"));
    }

    #[test]
    fn lumen_error_display_internal() {
        let e = LumenError::Internal("something broke".into());
        assert!(e.to_string().contains("internal error"));
    }

    #[test]
    fn lumen_error_display_storage() {
        let e = LumenError::Storage(StorageError::Corruption {
            message: "disk full".into(),
        });
        assert!(e.to_string().contains("disk full"));
    }

    #[test]
    fn lumen_error_display_durability() {
        let e = LumenError::Durability(DurabilityError {
            message: "fsync failed".into(),
        });
        assert!(e.to_string().contains("fsync failed"));
    }

    #[test]
    fn lumen_error_display_query() {
        let e = LumenError::Query(QueryError {
            message: "parse error".into(),
        });
        assert!(e.to_string().contains("parse error"));
    }

    #[test]
    fn lumen_error_source_schema() {
        use std::error::Error;
        let e = LumenError::Schema(SchemaError::NoSignalsDefined);
        assert!(e.source().is_some());
    }

    #[test]
    fn lumen_error_source_internal_is_none() {
        use std::error::Error;
        let e = LumenError::Internal("bug".into());
        assert!(e.source().is_none());
    }

    #[test]
    fn lumen_error_source_not_found_is_none() {
        use std::error::Error;
        let e = LumenError::NotFound {
            kind: EntityKind::User,
            id: EntityId::new(1),
        };
        assert!(e.source().is_none());
    }

    #[test]
    fn schema_error_converts_to_lumen_error() {
        let schema_err = SchemaError::NoSignalsDefined;
        let lumen_err: LumenError = schema_err.into();
        assert!(matches!(
            lumen_err,
            LumenError::Schema(SchemaError::NoSignalsDefined)
        ));
    }

    #[test]
    fn storage_error_converts_to_lumen_error() {
        let e = StorageError::Closed;
        let lumen_err: LumenError = e.into();
        assert!(matches!(lumen_err, LumenError::Storage(_)));
    }

    #[test]
    fn durability_error_converts_to_lumen_error() {
        let e = DurabilityError {
            message: "test".into(),
        };
        let lumen_err: LumenError = e.into();
        assert!(matches!(lumen_err, LumenError::Durability(_)));
    }

    #[test]
    fn query_error_converts_to_lumen_error() {
        let e = QueryError {
            message: "test".into(),
        };
        let lumen_err: LumenError = e.into();
        assert!(matches!(lumen_err, LumenError::Query(_)));
    }

    #[test]
    fn schema_error_display_all_variants() {
        assert_eq!(
            SchemaError::DuplicateSignalName("view".into()).to_string(),
            "duplicate signal name: 'view'"
        );
        assert_eq!(
            SchemaError::InvalidSignalName("BAD".into()).to_string(),
            "invalid signal name: 'BAD'"
        );
        assert!(
            SchemaError::InvalidHalfLife {
                signal_name: "s".into(),
                half_life_secs: 0.0,
            }
            .to_string()
            .contains("invalid half-life")
        );
        assert!(
            SchemaError::InvalidLifetime {
                signal_name: "s".into(),
                lifetime_secs: -1.0,
            }
            .to_string()
            .contains("invalid lifetime")
        );
        assert!(
            SchemaError::EmptyWindows {
                signal_name: "s".into()
            }
            .to_string()
            .contains("requires at least one window")
        );
        assert!(
            SchemaError::VelocityWithoutWindows {
                signal_name: "s".into()
            }
            .to_string()
            .contains("velocity requires")
        );
        assert_eq!(
            SchemaError::NoSignalsDefined.to_string(),
            "schema must define at least one signal"
        );
    }

    #[test]
    fn schema_error_eq() {
        assert_eq!(
            SchemaError::DuplicateSignalName("a".into()),
            SchemaError::DuplicateSignalName("a".into())
        );
        assert_ne!(
            SchemaError::DuplicateSignalName("a".into()),
            SchemaError::DuplicateSignalName("b".into())
        );
        assert_ne!(
            SchemaError::NoSignalsDefined,
            SchemaError::DuplicateSignalName("a".into())
        );
    }
}