use std::io::{Read, Seek, SeekFrom};

use byteorder::{ByteOrder, ReadBytesExt};
use linux_perf_event_reader::PerfEventAttr;

use crate::{perf_file::PerfFileSection, Error, ReadError};

/// `nr_cpus`
#[derive(Debug, Clone, Copy)]
pub struct NrCpus {
    /// CPUs not yet onlined
    pub nr_cpus_available: u32,
    pub nr_cpus_online: u32,
}

impl NrCpus {
    pub const STRUCT_SIZE: usize = 4 + 4;

    pub fn parse<R: Read, T: ByteOrder>(mut reader: R) -> Result<Self, std::io::Error> {
        let nr_cpus_available = reader.read_u32::<T>()?;
        let nr_cpus_online = reader.read_u32::<T>()?;
        Ok(Self {
            nr_cpus_available,
            nr_cpus_online,
        })
    }
}

/// The timestamps of the first and last sample.
#[derive(Debug, Clone, Copy)]
pub struct SampleTimeRange {
    pub first_sample_time: u64,
    pub last_sample_time: u64,
}

impl SampleTimeRange {
    pub const STRUCT_SIZE: usize = 8 + 8;

    pub fn parse<R: Read, T: ByteOrder>(mut reader: R) -> Result<Self, std::io::Error> {
        let first_sample_time = reader.read_u64::<T>()?;
        let last_sample_time = reader.read_u64::<T>()?;
        Ok(Self {
            first_sample_time,
            last_sample_time,
        })
    }
}

pub struct HeaderString(String);

impl HeaderString {
    /// Parse a string.
    pub fn parse<R: Read, T: ByteOrder>(mut reader: R) -> Result<Option<String>, std::io::Error> {
        let len = reader.read_u32::<T>()?;
        let mut s = vec![0; len as usize];
        reader.read_exact(&mut s)?;
        let actual_len = memchr::memchr(0, &s).unwrap_or(s.len());
        s.truncate(actual_len);
        Ok(String::from_utf8(s).ok())
    }
}

/// A single event attr with name and corresponding event IDs.
#[derive(Debug, Clone)]
pub struct AttributeDescription {
    pub attr: PerfEventAttr,
    pub name: Option<String>,
    pub event_ids: Vec<u64>,
}

impl AttributeDescription {
    /// Parse the HEADER_EVENT_DESC section into a Vec of AttributeDescription structs.
    pub fn parse_event_desc_section<R: Read, T: ByteOrder>(
        mut reader: R,
    ) -> Result<Vec<Self>, std::io::Error> {
        // ```c
        // struct {
        //   uint32_t nr; /* number of events */
        //   uint32_t attr_size; /* size of each perf_event_attr */
        //   struct {
        //     struct perf_event_attr attr;  /* size of attr_size */
        //     uint32_t nr_ids;
        //     struct perf_header_string event_string;
        //     uint64_t ids[nr_ids];
        //   } events[nr]; /* Variable length records */
        // };
        // ```
        let nr = reader.read_u32::<T>()?;
        let mut attributes = Vec::with_capacity(nr as usize);
        let attr_size = reader.read_u32::<T>()?;
        for _ in 0..nr {
            let attr = PerfEventAttr::parse::<_, T>(&mut reader, Some(attr_size))?;
            let nr_ids = reader.read_u32::<T>()?;
            let event_string = HeaderString::parse::<_, T>(&mut reader)?;
            let mut ids = Vec::with_capacity(nr_ids as usize);
            for _ in 0..nr_ids {
                ids.push(reader.read_u64::<T>()?);
            }
            attributes.push(AttributeDescription {
                attr,
                name: event_string,
                event_ids: ids,
            });
        }
        Ok(attributes)
    }

    /// Parse the event_types section into a Vec of AttributeDescription structs.
    /// This section was used in the past but is no longer used.
    /// Only call this function if event_types_section.size is non-zero.
    pub fn parse_event_types_section<C: Read + Seek, T: ByteOrder>(
        mut cursor: C,
        event_types_section: &PerfFileSection,
        attr_size: u64,
    ) -> Result<Vec<Self>, Error> {
        cursor.seek(SeekFrom::Start(event_types_section.offset))?;

        // Each entry in the event_types section is a PerfEventAttr followed by a PerfFileSection.
        let entry_size = attr_size + PerfFileSection::STRUCT_SIZE as u64;
        let entry_count = event_types_section.size / entry_size;
        let mut perf_event_event_type_info = Vec::with_capacity(entry_count as usize);
        for _ in 0..entry_count {
            let attr = PerfEventAttr::parse::<_, T>(&mut cursor, Some(attr_size as u32))
                .map_err(|_| ReadError::PerfEventAttr)?;
            let event_ids = PerfFileSection::parse::<_, T>(&mut cursor)?;
            perf_event_event_type_info.push((attr, event_ids));
        }

        // Read the lists of event IDs for each event type.
        let mut attributes = Vec::new();
        for (attr, section) in perf_event_event_type_info {
            cursor.seek(SeekFrom::Start(section.offset))?;
            // This section is just a list of u64 event IDs.
            let id_count = section.size / 8;
            let mut event_ids = Vec::with_capacity(id_count as usize);
            for _ in 0..id_count {
                event_ids.push(cursor.read_u64::<T>()?);
            }
            attributes.push(AttributeDescription {
                attr,
                name: None,
                event_ids,
            });
        }
        Ok(attributes)
    }

    /// Parse the attr section into a Vec of AttributeDescription structs.
    /// This section is used as a last resort because it does not have any
    /// information about event IDs. If multiple events are observed, we will
    /// not be able to know which event record belongs to which attr.
    pub fn parse_attr_section<C: Read + Seek, T: ByteOrder>(
        mut cursor: C,
        attr_section: &PerfFileSection,
        attr_size: u64,
    ) -> Result<Vec<Self>, Error> {
        cursor.seek(SeekFrom::Start(attr_section.offset))?;
        let attr_count = attr_section.size / attr_size;
        let mut attributes = Vec::with_capacity(attr_count as usize);
        for _ in 0..attr_count {
            let attr = PerfEventAttr::parse::<_, T>(&mut cursor, Some(attr_size as u32))
                .map_err(|_| ReadError::PerfEventAttr)?;
            attributes.push(AttributeDescription {
                attr,
                name: None,
                event_ids: vec![],
            });
        }
        Ok(attributes)
    }
}