synta 0.1.3

//! ASN.1 tag handling
//!
//! Tags consist of:
//! - Class: Universal, Application, Context-specific, Private
//! - Constructed/Primitive flag
//! - Number: The type identifier

use crate::error::{Error, Result};
#[cfg(all(not(feature = "std"), feature = "serde"))]
use alloc::string::String;
#[cfg(not(feature = "std"))]
use alloc::vec::Vec;

/// ASN.1 tag class
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(u8)]
pub enum TagClass {
    /// Universal (0b00)
    Universal = 0b00,
    /// Application (0b01)
    Application = 0b01,
    /// Context-specific (0b10)
    ContextSpecific = 0b10,
    /// Private (0b11)
    Private = 0b11,
}

impl TagClass {
    /// Create from byte value (upper 2 bits)
    #[inline(always)]
    pub fn from_byte(byte: u8) -> Self {
        match (byte >> 6) & 0b11 {
            0b00 => TagClass::Universal,
            0b01 => TagClass::Application,
            0b10 => TagClass::ContextSpecific,
            0b11 => TagClass::Private,
            _ => unreachable!(),
        }
    }

    /// Convert to byte value
    pub fn to_byte(self) -> u8 {
        (self as u8) << 6
    }
}

/// ASN.1 tag
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Tag {
    class: TagClass,
    constructed: bool,
    number: u32,
}

impl Tag {
    /// Create a new tag
    pub const fn new(class: TagClass, constructed: bool, number: u32) -> Self {
        Self {
            class,
            constructed,
            number,
        }
    }

    /// Create a universal tag
    pub const fn universal(number: u32) -> Self {
        Self::new(TagClass::Universal, false, number)
    }

    /// Create a universal constructed tag
    pub const fn universal_constructed(number: u32) -> Self {
        Self::new(TagClass::Universal, true, number)
    }

    /// Create a context-specific tag
    pub const fn context_specific(number: u32) -> Self {
        Self::new(TagClass::ContextSpecific, false, number)
    }

    /// Create a context-specific constructed tag
    pub const fn context_specific_constructed(number: u32) -> Self {
        Self::new(TagClass::ContextSpecific, true, number)
    }

    /// Create an application tag
    pub const fn application(number: u32) -> Self {
        Self::new(TagClass::Application, false, number)
    }

    /// Get the tag class
    pub const fn class(&self) -> TagClass {
        self.class
    }

    /// Check if the tag is constructed
    pub const fn is_constructed(&self) -> bool {
        self.constructed
    }

    /// Get the tag number
    pub const fn number(&self) -> u32 {
        self.number
    }

    /// Calculate the encoded length of this tag
    pub fn encoded_len(&self) -> usize {
        if self.number < 31 {
            1
        } else {
            // High tag number form
            1 + self.number_encoded_len()
        }
    }

    /// Calculate encoded length of tag number (for high tag number form)
    fn number_encoded_len(&self) -> usize {
        if self.number == 0 {
            1
        } else {
            let mut n = self.number;
            let mut len = 0;
            while n > 0 {
                len += 1;
                n >>= 7;
            }
            len
        }
    }

    /// Encode the tag to a buffer
    pub fn encode(&self, buffer: &mut Vec<u8>) -> Result<()> {
        if self.number < 31 {
            // Low tag number form (0-30)
            let byte = self.class.to_byte()
                | if self.constructed { 0b00100000 } else { 0 }
                | (self.number as u8);
            buffer.push(byte);
        } else {
            // High tag number form (>= 31)
            let first_byte =
                self.class.to_byte() | if self.constructed { 0b00100000 } else { 0 } | 0b00011111; // All five bits set
            buffer.push(first_byte);

            // Encode tag number in base-128, big-endian
            let mut encoded = Vec::new();
            let mut n = self.number;
            while n > 0 {
                encoded.push((n & 0x7F) as u8);
                n >>= 7;
            }

            // Reverse and set high bit on all but last byte
            for (i, byte) in encoded.iter().rev().enumerate() {
                if i < encoded.len() - 1 {
                    buffer.push(byte | 0x80);
                } else {
                    buffer.push(*byte);
                }
            }
        }
        Ok(())
    }

    /// Decode a tag from a byte slice
    /// Returns (tag, bytes_consumed)
    pub fn decode(bytes: &[u8], position: usize) -> Result<(Self, usize)> {
        if bytes.is_empty() {
            return Err(Error::UnexpectedEof { position });
        }

        let first_byte = bytes[0];
        let class = TagClass::from_byte(first_byte);
        let constructed = (first_byte & 0b00100000) != 0;
        let number_bits = first_byte & 0b00011111;

        if number_bits < 31 {
            // Low tag number form (fast path - most common)
            Ok((Tag::new(class, constructed, number_bits as u32), 1))
        } else {
            // High tag number form (cold path - rare)
            #[cfg(feature = "unchecked")]
            {
                Self::decode_high_tag_unchecked(bytes, class, constructed, first_byte, position)
            }
            #[cfg(not(feature = "unchecked"))]
            {
                Self::decode_high_tag_checked(bytes, class, constructed, first_byte, position)
            }
        }
    }

    /// Decode high tag number with validation (safe)
    #[cfg(not(feature = "unchecked"))]
    #[cold]
    #[inline(never)]
    fn decode_high_tag_checked(
        bytes: &[u8],
        class: TagClass,
        constructed: bool,
        first_byte: u8,
        position: usize,
    ) -> Result<(Self, usize)> {
        let mut number: u32 = 0;
        let mut bytes_read = 1;

        loop {
            if bytes_read >= bytes.len() {
                return Err(Error::UnexpectedEof {
                    position: position + bytes_read,
                });
            }

            let byte = bytes[bytes_read];
            bytes_read += 1;

            // Check for overflow
            if number > (u32::MAX >> 7) {
                return Err(Error::InvalidTag {
                    position,
                    byte: first_byte,
                });
            }

            number = (number << 7) | ((byte & 0x7F) as u32);

            // Check if this is the last byte (high bit not set)
            if (byte & 0x80) == 0 {
                break;
            }

            // Prevent infinite loops with overly long encodings
            if bytes_read > 5 {
                // 5 bytes is enough for u32
                return Err(Error::InvalidTag {
                    position,
                    byte: first_byte,
                });
            }
        }

        Ok((Tag::new(class, constructed, number), bytes_read))
    }

    /// Decode high tag number without validation (fast but assumes valid DER)
    #[cfg(feature = "unchecked")]
    #[cold]
    #[inline(never)]
    fn decode_high_tag_unchecked(
        bytes: &[u8],
        class: TagClass,
        constructed: bool,
        _first_byte: u8,
        _position: usize,
    ) -> Result<(Self, usize)> {
        let mut number: u32 = 0;
        let mut bytes_read = 1;

        loop {
            let byte = bytes[bytes_read];
            bytes_read += 1;

            // Skip overflow check - assume valid DER
            number = (number << 7) | ((byte & 0x7F) as u32);

            // Check if this is the last byte (high bit not set)
            if (byte & 0x80) == 0 {
                break;
            }

            // Skip length check - assume valid DER
        }

        Ok((Tag::new(class, constructed, number), bytes_read))
    }
}

// Universal tag numbers as constants

/// Tag number for BOOLEAN type (tag 1)
pub const TAG_BOOLEAN: u32 = 1;

/// Tag number for INTEGER type (tag 2)
pub const TAG_INTEGER: u32 = 2;

/// Tag number for BIT STRING type (tag 3)
pub const TAG_BIT_STRING: u32 = 3;

/// Tag number for OCTET STRING type (tag 4)
pub const TAG_OCTET_STRING: u32 = 4;

/// Tag number for NULL type (tag 5)
pub const TAG_NULL: u32 = 5;

/// Tag number for OBJECT IDENTIFIER type (tag 6)
pub const TAG_OBJECT_IDENTIFIER: u32 = 6;

/// Tag number for REAL type (tag 9)
pub const TAG_REAL: u32 = 9;

/// Tag number for ENUMERATED type (tag 10)
pub const TAG_ENUMERATED: u32 = 10;

/// Tag number for UTF8String type (tag 12)
pub const TAG_UTF8_STRING: u32 = 12;

/// Tag number for SEQUENCE and SEQUENCE OF types (tag 16)
pub const TAG_SEQUENCE: u32 = 16;

/// Tag number for SET and SET OF types (tag 17)
pub const TAG_SET: u32 = 17;

/// Tag number for PrintableString type (tag 19)
pub const TAG_PRINTABLE_STRING: u32 = 19;

/// Tag number for IA5String type (tag 22)
pub const TAG_IA5_STRING: u32 = 22;

/// Tag number for UTCTime type (tag 23)
pub const TAG_UTC_TIME: u32 = 23;

/// Tag number for GeneralizedTime type (tag 24)
pub const TAG_GENERALIZED_TIME: u32 = 24;

/// Tag number for VisibleString type (tag 26) — printable ASCII subset
pub const TAG_VISIBLE_STRING: u32 = 26;

/// Tag number for GeneralString type (tag 27) — 8-bit character set, treated as Latin-1
pub const TAG_GENERAL_STRING: u32 = 27;

/// Tag number for UniversalString type (tag 28) — UCS-4 (UTF-32 big-endian)
pub const TAG_UNIVERSAL_STRING: u32 = 28;

/// Tag number for BMPString type (tag 30) — UCS-2 (UTF-16 big-endian, BMP only)
pub const TAG_BMP_STRING: u32 = 30;

/// Tag number for NumericString type (tag 18) — digits and space
pub const TAG_NUMERIC_STRING: u32 = 18;

/// Tag number for TeletexString / T61String type (tag 20) — arbitrary 8-bit bytes
pub const TAG_TELETEX_STRING: u32 = 20;

// ---- serde support ----

#[cfg(feature = "serde")]
impl serde::Serialize for TagClass {
    fn serialize<S: serde::Serializer>(&self, s: S) -> core::result::Result<S::Ok, S::Error> {
        s.serialize_str(match self {
            TagClass::Universal => "Universal",
            TagClass::Application => "Application",
            TagClass::ContextSpecific => "ContextSpecific",
            TagClass::Private => "Private",
        })
    }
}

#[cfg(feature = "serde")]
impl<'de> serde::Deserialize<'de> for TagClass {
    fn deserialize<D: serde::Deserializer<'de>>(d: D) -> core::result::Result<Self, D::Error> {
        struct V;
        impl<'de> serde::de::Visitor<'de> for V {
            type Value = TagClass;
            fn expecting(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
                write!(
                    f,
                    "one of \"Universal\", \"Application\", \"ContextSpecific\", \"Private\""
                )
            }
            fn visit_str<E: serde::de::Error>(self, v: &str) -> core::result::Result<TagClass, E> {
                match v {
                    "Universal" => Ok(TagClass::Universal),
                    "Application" => Ok(TagClass::Application),
                    "ContextSpecific" => Ok(TagClass::ContextSpecific),
                    "Private" => Ok(TagClass::Private),
                    _ => Err(E::unknown_variant(
                        v,
                        &["Universal", "Application", "ContextSpecific", "Private"],
                    )),
                }
            }
        }
        d.deserialize_str(V)
    }
}

#[cfg(feature = "serde")]
impl serde::Serialize for Tag {
    fn serialize<S: serde::Serializer>(&self, s: S) -> core::result::Result<S::Ok, S::Error> {
        use serde::ser::SerializeStruct;
        let mut st = s.serialize_struct("Tag", 3)?;
        st.serialize_field("class", &self.class())?;
        st.serialize_field("constructed", &self.is_constructed())?;
        st.serialize_field("number", &self.number())?;
        st.end()
    }
}

#[cfg(feature = "serde")]
impl<'de> serde::Deserialize<'de> for Tag {
    fn deserialize<D: serde::Deserializer<'de>>(d: D) -> core::result::Result<Self, D::Error> {
        struct TagVisitor;
        impl<'de> serde::de::Visitor<'de> for TagVisitor {
            type Value = Tag;
            fn expecting(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
                write!(f, "a Tag with class, constructed, and number fields")
            }
            fn visit_map<A: serde::de::MapAccess<'de>>(
                self,
                mut map: A,
            ) -> core::result::Result<Tag, A::Error> {
                let mut class: Option<TagClass> = None;
                let mut constructed: Option<bool> = None;
                let mut number: Option<u32> = None;
                while let Some(key) = map.next_key::<String>()? {
                    match key.as_str() {
                        "class" => class = Some(map.next_value()?),
                        "constructed" => constructed = Some(map.next_value()?),
                        "number" => number = Some(map.next_value()?),
                        _ => {
                            let _ = map.next_value::<serde::de::IgnoredAny>()?;
                        }
                    }
                }
                let class = class.ok_or_else(|| serde::de::Error::missing_field("class"))?;
                let constructed =
                    constructed.ok_or_else(|| serde::de::Error::missing_field("constructed"))?;
                let number = number.ok_or_else(|| serde::de::Error::missing_field("number"))?;
                Ok(Tag::new(class, constructed, number))
            }
        }
        d.deserialize_map(TagVisitor)
    }
}