use std::fmt;

use serde_json::{Map, Number, Value};
use time::{OffsetDateTime, format_description::well_known::Rfc3339};
use tracing::{Event, Subscriber, field::Visit};
use tracing_subscriber::{
    EnvFilter, Layer,
    filter::filter_fn,
    fmt::{FmtContext, FormatEvent, FormatFields, MakeWriter, format::Writer},
    layer::SubscriberExt,
    registry::LookupSpan,
};

use crate::{
    ObservabilityConfig, ObservabilityInitError, RedactionLimits, ServiceIdentity,
    propagation::current_trace_id,
    redaction::{redact_value, truncate_string},
    schema::LogEnvelope,
};

pub fn build_subscriber<W>(
    config: ObservabilityConfig,
    writer: W,
) -> Result<impl Subscriber + Send + Sync, ObservabilityInitError>
where
    W: for<'writer> MakeWriter<'writer> + Send + Sync + 'static,
{
    build_subscriber_with_tracer(config, writer, None)
}

pub(crate) fn build_subscriber_with_tracer<W>(
    config: ObservabilityConfig,
    writer: W,
    tracer: Option<opentelemetry_sdk::trace::SdkTracer>,
) -> Result<impl Subscriber + Send + Sync, ObservabilityInitError>
where
    W: for<'writer> MakeWriter<'writer> + Send + Sync + 'static,
{
    let (identity, filter, limits) = config.into_parts();
    limits.validate()?;
    let filter = EnvFilter::try_new(filter).map_err(|_| ObservabilityInitError::InvalidFilter)?;
    let formatter = JsonEventFormatter::new(identity, limits);
    let fmt_layer = tracing_subscriber::fmt::layer()
        .with_ansi(false)
        .event_format(formatter)
        .with_writer(writer)
        .with_filter(filter);
    let otel_layer = tracer.map(|tracer| {
        tracing_opentelemetry::layer()
            .with_tracer(tracer)
            .with_filter(filter_fn(|metadata| {
                metadata.is_span() && metadata.target() == "crank::trace"
            }))
    });

    Ok(tracing_subscriber::registry()
        .with(fmt_layer)
        .with(otel_layer))
}

#[derive(Clone, Debug)]
struct JsonEventFormatter {
    identity: ServiceIdentity,
    limits: RedactionLimits,
}

impl JsonEventFormatter {
    fn new(identity: ServiceIdentity, limits: RedactionLimits) -> Self {
        Self { identity, limits }
    }

    fn envelope(&self, event: &Event<'_>) -> Result<LogEnvelope, fmt::Error> {
        let metadata = event.metadata();
        let mut visitor = JsonFieldVisitor::default();
        event.record(&mut visitor);
        let mut raw_fields = visitor.fields;
        let request_id = take_correlation_id(&mut raw_fields, "request_id")
            .map(|value| truncate_string(&value, self.limits.max_string_bytes));
        let trace_id = take_correlation_id(&mut raw_fields, "trace_id")
            .or_else(current_trace_id)
            .map(|value| truncate_string(&value, self.limits.max_string_bytes));
        let cleaned = redact_value(&Value::Object(raw_fields), self.limits);
        let fields = cleaned.as_object().cloned().unwrap_or_default();
        let timestamp = OffsetDateTime::now_utc()
            .format(&Rfc3339)
            .map_err(|_| fmt::Error)?;

        Ok(LogEnvelope {
            timestamp,
            level: metadata.level().as_str().to_owned(),
            service: self.identity.service().to_owned(),
            version: self.identity.version().to_owned(),
            environment: self.identity.environment().to_owned(),
            target: truncate_string(metadata.target(), self.limits.max_string_bytes),
            event: truncate_string(metadata.name(), self.limits.max_string_bytes),
            request_id,
            trace_id,
            fields,
        })
    }

    fn serialize_bounded(&self, mut envelope: LogEnvelope) -> Result<String, fmt::Error> {
        let line_budget = self.limits.max_event_bytes.saturating_sub(1);
        let serialized = serde_json::to_string(&envelope).map_err(|_| fmt::Error)?;
        if serialized.len() <= line_budget {
            return Ok(serialized);
        }

        envelope.fields = Map::from_iter([("truncated".to_owned(), Value::Bool(true))]);
        let fallback_string_limit = self.limits.max_string_bytes.min(64);
        envelope.target = truncate_string(&envelope.target, fallback_string_limit);
        envelope.event = truncate_string(&envelope.event, fallback_string_limit);
        envelope.request_id = envelope
            .request_id
            .map(|value| truncate_string(&value, fallback_string_limit));
        envelope.trace_id = envelope
            .trace_id
            .map(|value| truncate_string(&value, fallback_string_limit));
        let serialized = serde_json::to_string(&envelope).map_err(|_| fmt::Error)?;
        if serialized.len() <= line_budget {
            return Ok(serialized);
        }

        envelope.request_id = None;
        envelope.trace_id = None;
        envelope.target = truncate_string(&envelope.target, 16);
        envelope.event = truncate_string(&envelope.event, 16);
        let serialized = serde_json::to_string(&envelope).map_err(|_| fmt::Error)?;
        (serialized.len() <= line_budget)
            .then_some(serialized)
            .ok_or(fmt::Error)
    }
}

impl<S, N> FormatEvent<S, N> for JsonEventFormatter
where
    S: Subscriber + for<'lookup> LookupSpan<'lookup>,
    N: for<'writer> FormatFields<'writer> + 'static,
{
    fn format_event(
        &self,
        _ctx: &FmtContext<'_, S, N>,
        mut writer: Writer<'_>,
        event: &Event<'_>,
    ) -> fmt::Result {
        let serialized = self.serialize_bounded(self.envelope(event)?)?;
        writer.write_str(&serialized)?;
        writer.write_char('\n')
    }
}

#[derive(Default)]
struct JsonFieldVisitor {
    fields: Map<String, Value>,
}

impl JsonFieldVisitor {
    fn insert(&mut self, field: &tracing::field::Field, value: Value) {
        self.fields.insert(field.name().to_owned(), value);
    }
}

impl Visit for JsonFieldVisitor {
    fn record_i64(&mut self, field: &tracing::field::Field, value: i64) {
        self.insert(field, Value::Number(value.into()));
    }

    fn record_u64(&mut self, field: &tracing::field::Field, value: u64) {
        self.insert(field, Value::Number(value.into()));
    }

    fn record_bool(&mut self, field: &tracing::field::Field, value: bool) {
        self.insert(field, Value::Bool(value));
    }

    fn record_f64(&mut self, field: &tracing::field::Field, value: f64) {
        let value = Number::from_f64(value)
            .map(Value::Number)
            .unwrap_or(Value::Null);
        self.insert(field, value);
    }

    fn record_str(&mut self, field: &tracing::field::Field, value: &str) {
        self.insert(field, Value::String(value.to_owned()));
    }

    fn record_debug(&mut self, field: &tracing::field::Field, value: &dyn fmt::Debug) {
        let rendered = format!("{value:?}");
        let value = if is_correlation_field(field.name()) {
            Value::String(debug_scalar(&rendered))
        } else {
            match serde_json::from_str(&rendered) {
                Ok(value @ (Value::Object(_) | Value::Array(_))) => value,
                _ if field.name() == "message" || is_safe_display_scalar(&rendered) => {
                    Value::String(rendered)
                }
                _ => Value::String(crate::REDACTED_MARKER.to_owned()),
            }
        };
        self.insert(field, value);
    }
}

fn take_correlation_id(fields: &mut Map<String, Value>, name: &str) -> Option<String> {
    let value = fields.remove(name)?;
    let value = match value {
        Value::String(value) => value,
        Value::Number(value) => value.to_string(),
        Value::Bool(value) => value.to_string(),
        Value::Null | Value::Array(_) | Value::Object(_) => return None,
    };
    (!value.is_empty()).then_some(value)
}

fn is_correlation_field(name: &str) -> bool {
    matches!(name, "request_id" | "trace_id" | "correlation_id")
}

fn debug_scalar(rendered: &str) -> String {
    serde_json::from_str::<String>(rendered).unwrap_or_else(|_| rendered.to_owned())
}

fn is_safe_display_scalar(rendered: &str) -> bool {
    !rendered.is_empty()
        && rendered.bytes().all(|byte| {
            byte.is_ascii_alphanumeric()
                || matches!(byte, b'-' | b'_' | b'.' | b':' | b'/' | b'+' | b'@')
        })
}

#[cfg(test)]
mod tests {
    use std::{
        io,
        sync::{Arc, Mutex},
    };

    use opentelemetry::{global, trace::TracerProvider as _};
    use opentelemetry_sdk::{
        error::OTelSdkResult,
        propagation::TraceContextPropagator,
        trace::{SdkTracerProvider, SpanData, SpanExporter},
    };
    use tracing::info;

    use super::build_subscriber_with_tracer;
    use crate::{
        ObservabilityConfig, RedactionLimits, ServiceIdentity, inject_current_trace_context,
    };

    #[test]
    fn trace_spans_ignore_the_log_level_filter() {
        global::set_text_map_propagator(TraceContextPropagator::new());
        let provider = SdkTracerProvider::builder().build();
        let tracer = provider.tracer("trace-filter-test");
        let subscriber = build_subscriber_with_tracer(test_config("warn"), io::sink, Some(tracer))
            .expect("subscriber must build");
        let dispatch = tracing::Dispatch::new(subscriber);
        let _dispatch_guard = tracing::dispatcher::set_default(&dispatch);
        let span = tracing::info_span!(target: "crank::trace", "http.request");
        let _span_guard = span.enter();
        let mut headers = axum::http::HeaderMap::new();

        assert!(inject_current_trace_context(&mut headers));
        assert!(headers.contains_key("traceparent"));
    }

    #[test]
    fn otel_layer_does_not_export_events() {
        let exported = Arc::new(Mutex::new(Vec::new()));
        let provider = SdkTracerProvider::builder()
            .with_simple_exporter(CapturingExporter(Arc::clone(&exported)))
            .build();
        let tracer = provider.tracer("event-filter-test");
        let subscriber = build_subscriber_with_tracer(test_config("info"), io::sink, Some(tracer))
            .expect("subscriber must build");
        let dispatch = tracing::Dispatch::new(subscriber);

        tracing::dispatcher::with_default(&dispatch, || {
            let span = tracing::info_span!(target: "crank::trace", "http.request");
            let _span_guard = span.enter();
            info!(password = "canary-secret", "sensitive event");
        });
        provider.force_flush().expect("span must be exported");

        let spans = exported.lock().expect("capture lock");
        assert_eq!(spans.len(), 1);
        assert!(spans[0].events.is_empty());
        assert!(
            !format!("{:?}", spans[0])
                .as_bytes()
                .windows(b"canary-secret".len())
                .any(|window| window == b"canary-secret")
        );
    }

    fn test_config(filter: &str) -> ObservabilityConfig {
        ObservabilityConfig::new(
            ServiceIdentity::try_new("admin-api", "test", "test").unwrap(),
            filter,
            RedactionLimits::default(),
        )
    }

    #[derive(Clone, Debug)]
    struct CapturingExporter(Arc<Mutex<Vec<SpanData>>>);

    impl SpanExporter for CapturingExporter {
        async fn export(&self, batch: Vec<SpanData>) -> OTelSdkResult {
            self.0.lock().expect("capture lock").extend(batch);
            Ok(())
        }
    }
}