// Moved from apps/selfhost-compiler/boxes/parser/expr/parser_peek_box.hako
// ParserPeekBox — peek expression parser (peek <expr> { "label" => <expr>, ..., else => <expr> })
// Responsibility: Parse peek expressions (pattern-matching syntax)
// API: parse(src, i, ctx) -> JSON string

static box ParserPeekBox {
  parse(src, i, ctx) {
    // ctx is ParserBox for delegation
    local j = i
    local n = src.length()

    // Parse scrutinee expression
    local scr = ctx.parse_expr2(src, j)
    j = ctx.gpos_get()
    j = ctx.skip_ws(src, j)

    // Enter arms block
    if src.substring(j, j+1) == "{" { j = j + 1 }
    j = ctx.skip_ws(src, j)

    local arms_json = "["
    local first_arm = 1
    local else_json = null
    local contp = 1
    local guardp = 0
    local maxp = 400000

    loop(contp == 1) {
      if guardp > maxp { contp = 0 } else { guardp = guardp + 1 }
      j = ctx.skip_ws(src, j)

      if j >= n {
        contp = 0
      } else {
        if src.substring(j, j+1) == "}" {
          j = j + 1
          contp = 0
        } else {
          // else arm or labeled arm
          if ctx.starts_with_kw(src, j, "else") == 1 {
            j = j + 4
            j = ctx.skip_ws(src, j)
            if src.substring(j, j+2) == "=>" { j = j + 2 }
            j = ctx.skip_ws(src, j)

            // else body may be a block or bare expr
            if src.substring(j, j+1) == "{" {
              j = j + 1
              j = ctx.skip_ws(src, j)
              else_json = ctx.parse_expr2(src, j)
              j = ctx.gpos_get()
              j = ctx.skip_ws(src, j)
              if src.substring(j, j+1) == "}" { j = j + 1 }
            } else {
              else_json = ctx.parse_expr2(src, j)
              j = ctx.gpos_get()
            }
          } else {
            // labeled arm: string literal label
            if src.substring(j, j+1) != "\"" {
              // degrade safely to avoid infinite loop
              j = j + 1
              continue
            }

            local label_raw = ctx.read_string_lit(src, j)
            j = ctx.gpos_get()
            j = ctx.skip_ws(src, j)
            if src.substring(j, j+2) == "=>" { j = j + 2 }
            j = ctx.skip_ws(src, j)

            // arm expr: block or bare expr
            local expr_json = "{\"type\":\"Int\",\"value\":0}"
            if src.substring(j, j+1) == "{" {
              j = j + 1
              j = ctx.skip_ws(src, j)
              expr_json = ctx.parse_expr2(src, j)
              j = ctx.gpos_get()
              j = ctx.skip_ws(src, j)
              if src.substring(j, j+1) == "}" { j = j + 1 }
            } else {
              expr_json = ctx.parse_expr2(src, j)
              j = ctx.gpos_get()
            }

            local arm_json = "{\"label\":\"" + ctx.esc_json(label_raw) + "\",\"expr\":" + expr_json + "}"
            if first_arm == 1 {
              arms_json = arms_json + arm_json
              first_arm = 0
            } else {
              arms_json = arms_json + "," + arm_json
            }
          }
        }
      }
    }

    arms_json = arms_json + "]"
    if else_json == null { else_json = "{\"type\":\"Null\"}" }
    ctx.gpos_set(j)
    return "{\"type\":\"Peek\",\"scrutinee\":" + scr + ",\"arms\":" + arms_json + ",\"else\":" + else_json + "}"
  }
}