ValueScript/valuescript_vm/src/string_methods.rs

use std::rc::Rc;

use crate::{helpers::to_wrapping_index, native_function::NativeFunction, vs_value::Val, ValTrait};

pub fn op_sub_string(string_data: &Rc<String>, subscript: &Val) -> Val {
  let right_index = match subscript.to_index() {
    None => {
      let method = subscript.val_to_string();
      let method_str = method.as_str();

      return match method_str {
        "length" => Val::Number(string_data.as_bytes().len() as f64),
        _ => get_string_method(method_str),
      };
    }
    Some(i) => i,
  };

  let string_bytes = string_data.as_bytes();

  if right_index >= string_bytes.len() {
    return Val::Undefined;
  }

  match unicode_at(string_bytes, right_index) {
    Some(string) => Val::String(Rc::new(string)),
    None => Val::String(Rc::new(String::from(""))),
  }
}

pub fn get_string_method(method: &str) -> Val {
  match method {
    "at" => Val::Static(&AT),
    "charAt" => Val::Static(&CHAR_AT),
    "charCodeAt" => Val::Static(&CHAR_CODE_AT),
    "codePointAt" => Val::Static(&CODE_POINT_AT),
    _ => Val::Undefined,
  }
}

static AT: NativeFunction = NativeFunction {
  fn_: |this: &mut Val, params: Vec<Val>| -> Val {
    match this {
      Val::String(string_data) => {
        let string_bytes = string_data.as_bytes();

        let index = match to_wrapping_index(params.get(0), string_bytes.len()) {
          None => return Val::Undefined,
          Some(i) => i,
        };

        match unicode_at(string_bytes, index) {
          Some(string) => Val::String(Rc::new(string)),
          None => Val::String(Rc::new(String::from(""))),
        }
      }
      _ => std::panic!("Not implemented: exceptions/string indirection"),
    }
  },
};

static CHAR_AT: NativeFunction = NativeFunction {
  fn_: |this: &mut Val, params: Vec<Val>| -> Val {
    match this {
      Val::String(string_data) => match byte_at(string_data, params.get(0)) {
        Some(byte) => match std::char::from_u32(byte as u32) {
          Some(c) => Val::String(Rc::new(c.to_string())),
          None => Val::String(Rc::new(String::from(""))),
        },
        None => Val::String(Rc::new(String::from(""))),
      },
      _ => std::panic!("Not implemented: exceptions/string indirection"),
    }
  },
};

static CHAR_CODE_AT: NativeFunction = NativeFunction {
  fn_: |this: &mut Val, params: Vec<Val>| -> Val {
    match this {
      Val::String(string_data) => match byte_at(string_data, params.get(0)) {
        Some(byte) => Val::Number(byte as f64),
        None => Val::Number(std::f64::NAN),
      },
      _ => std::panic!("Not implemented: exceptions/string indirection"),
    }
  },
};

static CODE_POINT_AT: NativeFunction = NativeFunction {
  fn_: |this: &mut Val, params: Vec<Val>| -> Val {
    match this {
      Val::String(string_data) => {
        let string_bytes = string_data.as_bytes();

        let index = match params.get(0) {
          Some(i) => match i.to_index() {
            None => return Val::Undefined,
            Some(i) => i,
          },
          _ => return Val::Undefined,
        };

        match code_point_at(string_bytes, index) {
          Some(code_point) => Val::Number(code_point as f64),
          None => Val::Undefined,
        }
      }
      _ => std::panic!("Not implemented: exceptions/string indirection"),
    }
  },
};

fn unicode_at(bytes: &[u8], index: usize) -> Option<String> {
  match code_point_at(bytes, index) {
    Some(code_point) => Some(
      std::char::from_u32(code_point)
        .expect("Invalid code point") // TODO: Find out if this is reachable and what to do about it
        .to_string(),
    ),
    None => None,
  }
}

fn code_point_at(bytes: &[u8], index: usize) -> Option<u32> {
  if index >= bytes.len() {
    return None;
  }

  let byte = bytes[index];

  let leading_ones = byte.leading_ones() as usize;

  if leading_ones == 0 {
    return Some(byte as u32);
  }

  if leading_ones == 1 || leading_ones > 4 || index + leading_ones > bytes.len() {
    return None;
  }

  let mut value = (byte & (0x7F >> leading_ones)) as u32;

  for i in 1..leading_ones {
    let next_byte = bytes[index + i];

    if next_byte.leading_ones() != 1 {
      return None;
    }

    value = (value << 6) | (next_byte & 0x3F) as u32;
  }

  Some(value)
}

fn byte_at(string: &String, index_param: Option<&Val>) -> Option<u8> {
  let mut index = match index_param {
    Some(i) => i.to_number(),
    _ => 0 as f64,
  };

  if index.is_nan() {
    index = 0 as f64;
  }

  index = index.trunc();

  if index < 0_f64 {
    return None;
  }

  let index_usize = index as usize;

  let string_bytes = string.as_bytes();

  if index_usize >= string_bytes.len() {
    return None;
  }

  Some(string_bytes[index_usize])
}