// ny_parser_v0 — Stage 1 MVP: Ny -> JSON v0 (minimal)
// Supports: return <expr>;  expr ::= term (('+'|'-') term)*
//          term  ::= factor (('*'|'/') factor)*
//          factor::= INT | STRING | '(' expr ')'

static box Main {
  // --- Utilities ---
  is_digit(ch) { return ch >= "0" && ch <= "9" }
  is_space(ch) {
    return ch == " " || ch == "\t" || ch == "\n" || ch == "\r"
  }

  esc_json(s) {
    // escape backslash and quote for JSON strings
    local out = ""
    local i = 0
    local n = s.length()
    loop(i < n) {
      local ch = s.substring(i, i+1)
      if ch == "\\" { out = out + "\\\\" } else {
        if ch == "\"" { out = out + "\\\"" } else { out = out + ch }
      }
      i = i + 1
    }
    return out
  }

  // Cursor helpers over source string
  skip_ws(src, i) {
    local n = src.length()
    loop(i < n && me.is_space(src.substring(i, i+1))) { i = i + 1 }
    return i
  }

  // (helper match removed; inline checks in parse_program)

  parse_number(src, i) {
    // returns (json, next_i)
    local n = src.length()
    local j = i
    loop(j < n && me.is_digit(src.substring(j, j+1))) { j = j + 1 }
    local s = src.substring(i, j)
    local json = "{\"type\":\"Int\",\"value\":" + s + "}"
    return json + "@" + j // pack result with '@' separator
  }

  parse_string(src, i) {
    local n = src.length()
    local j = i + 1 // skip opening quote
    local out = ""
    loop(j < n) {
      local ch = src.substring(j, j+1)
      if ch == "\"" {
        j = j + 1
        local json0 = "{\"type\":\"Str\",\"value\":\"" + me.esc_json(out) + "\"}"
        return json0 + "@" + j
      }
      if ch == "\\" && j + 1 < n {
        local nx = src.substring(j+1, j+2)
        // minimal escapes (\" and \\)
        if nx == "\"" { out = out + "\"" } else { if nx == "\\" { out = out + "\\" } else { out = out + nx } }
        j = j + 2
      } else {
        out = out + ch
        j = j + 1
      }
    }
    // Unterminated string (fallback)
    local json = "{\"type\":\"Str\",\"value\":\"" + me.esc_json(out) + "\"}"
    return json + "@" + j
  }

  // Recursive descent
  parse_factor(src, i) {
    // skip ws
    local nsrc = src.length()
    loop(i < nsrc && me.is_space(src.substring(i, i+1))) { i = i + 1 }
    local ch = src.substring(i, i+1)
    if ch == "(" {
      // (expr)
      local p = me.parse_expr(src, i + 1)
      local at = p.lastIndexOf("@")
      local ej = p.substring(0, at)
      local j = me.to_int(p.substring(at+1, p.length()))
      // skip ws
      local n2 = src.length()
      loop(j < n2 && me.is_space(src.substring(j, j+1))) { j = j + 1 }
      if src.substring(j, j+1) == ")" { j = j + 1 }
      return ej + "@" + j
    }
    if ch == "\"" { return me.parse_string(src, i) }
    // number
    return me.parse_number(src, i)
  }

  parse_term(src, i) {
    local p = me.parse_factor(src, i)
    local at = p.lastIndexOf("@")
    local lhs = p.substring(0, at)
    local j = me.to_int(p.substring(at+1, p.length()))
    local cont = 1
    loop(cont == 1) {
      // skip ws
      local n3 = src.length()
      loop(j < n3 && me.is_space(src.substring(j, j+1))) { j = j + 1 }
      if j >= src.length() { cont = 0 } else {
        local op = src.substring(j, j+1)
        if op != "*" && op != "/" { cont = 0 } else {
          // parse rhs
          local q = me.parse_factor(src, j+1)
          local at2 = q.lastIndexOf("@")
          local rhs = q.substring(0, at2)
          j = me.to_int(q.substring(at2+1, q.length()))
          lhs = "{\"type\":\"Binary\",\"op\":\"" + op + "\",\"lhs\":" + lhs + ",\"rhs\":" + rhs + "}"
        }
      }
    }
    return lhs + "@" + j
  }

  parse_expr(src, i) {
    local p = me.parse_term(src, i)
    local at = p.lastIndexOf("@")
    local lhs = p.substring(0, at)
    local j = me.to_int(p.substring(at+1, p.length()))
    local cont = 1
    loop(cont == 1) {
      // skip ws
      local n4 = src.length()
      loop(j < n4 && me.is_space(src.substring(j, j+1))) { j = j + 1 }
      if j >= src.length() { cont = 0 } else {
        local op = src.substring(j, j+1)
        if op != "+" && op != "-" { cont = 0 } else {
          // parse rhs
          local q = me.parse_term(src, j+1)
          local at2 = q.lastIndexOf("@")
          local rhs = q.substring(0, at2)
          j = me.to_int(q.substring(at2+1, q.length()))
          lhs = "{\"type\":\"Binary\",\"op\":\"" + op + "\",\"lhs\":" + lhs + ",\"rhs\":" + rhs + "}"
        }
      }
    }
    return lhs + "@" + j
  }

  to_int(s) {
    // parse decimal int from string s
    // 簡易に桁を読む（ここでは利用側が整数のみで使う）
    // ただしここは内部専用で index を取り出すだけなので、s は数字のみ想定
    // 実装簡略化のため、長さ0なら0、それ以外は手動で畳み込み
    local n = s.length()
    if n == 0 { return 0 }
    local i = 0
    local acc = 0
    loop(i < n) {
      local d = s.substring(i, i+1)
      local dv = 0
      if d == "1" { dv = 1 } else {
        if d == "2" { dv = 2 } else {
          if d == "3" { dv = 3 } else {
            if d == "4" { dv = 4 } else {
              if d == "5" { dv = 5 } else {
                if d == "6" { dv = 6 } else {
                  if d == "7" { dv = 7 } else {
                    if d == "8" { dv = 8 } else {
                      if d == "9" { dv = 9 }
                    }
                  }
                }
              }
            }
          }
        }
      }
      acc = acc * 10 + dv
      i = i + 1
    }
    return acc
  }

  parse_program(src) {
    // optional leading ws + optional 'return'
    // skip leading ws
    local i = 0
    local n0 = src.length()
    loop(i < n0 && me.is_space(src.substring(i, i+1))) { i = i + 1 }
    local j = i
    local n = src.length()
    if i + 6 <= n && src.substring(i, i+6) == "return" { j = i + 6 }
    local p = me.parse_expr(src, j)
    local at = p.lastIndexOf("@")
    local ej = p.substring(0, at)
    local body = "[{\"type\":\"Return\",\"expr\":" + ej + "}]"
    return "{\"version\":0,\"kind\":\"Program\",\"body\":" + body + "}"
  }

  read_all(path) {
    local fb = new FileBox()
    fb.open(path, "r")
    local s = fb.read()
    fb.close()
    return s
  }

  main(args) {
    // usage: nyash --backend vm apps/selfhost/parser/ny_parser_v0/main.nyash
    // Input source is read from tmp/ny_parser_input.ny (written by wrapper script)
    local console = new ConsoleBox()
    local src = null
    local default_path = "tmp/ny_parser_input.ny"
    src = me.read_all(default_path)
    if src == null { src = "return 1+2*3" }
    local json = me.parse_program(src)
    console.println(json)
    return 0
  }
}