-- frontmatter.lua — JSON-Frontmatter parser for manifest.launcher.md
-- Splits on the first standalone '---' line. Everything before goes to
-- depf._json_decode; everything after (after a single LF) is body.
-- Loud-Error semantics: parse failure returns nil + err-string.

local M = {}

-- depf is required lazily so this file can be dofile'd into a test-lib
-- context where lib-management.dep-fetcher is not on the dep list. The
-- launcher main module does require it eagerly; the test-lib monkeys
-- a stub decoder into _G before calling fm.parse.
local function get_decoder()
    if _G.LAUNCHER_FRONTMATTER_DECODER then
        return _G.LAUNCHER_FRONTMATTER_DECODER
    end
    local ok, depf = pcall(require, "lib-management.dep-fetcher")
    if ok and depf and depf._json_decode then return depf._json_decode end
    return nil
end

-- Splits the content at the first line equal to '---'. The first such
-- line ends the JSON region; lines before it are JSON, lines after
-- are body. Both regions are LF-joined back into strings.
local function split_at_separator(content)
    local lines = {}
    for line in (content .. "\n"):gmatch("([^\n]*)\n") do
        lines[#lines + 1] = line
    end
    -- A valid file starts with '---' (no leading whitespace).
    if lines[1] ~= "---" then return nil end
    local sep_idx = nil
    for i = 2, #lines do
        if lines[i] == "---" then sep_idx = i; break end
    end
    if not sep_idx then return nil end
    local json_lines = {}
    for i = 2, sep_idx - 1 do json_lines[#json_lines + 1] = lines[i] end
    local body_lines = {}
    for i = sep_idx + 1, #lines do body_lines[#body_lines + 1] = lines[i] end
    -- Drop the trailing empty entry produced by the final LF appended above,
    -- so the body matches user expectation ('body text' rather than 'body text\n').
    if #body_lines > 0 and body_lines[#body_lines] == "" then
        body_lines[#body_lines] = nil
    end
    return table.concat(json_lines, "\n"), table.concat(body_lines, "\n")
end

function M.parse(content)
    if type(content) ~= "string" then
        return nil, nil, "frontmatter.parse: content must be string"
    end
    local json_str, body = split_at_separator(content)
    if not json_str then
        -- No frontmatter — treat the whole content as body, empty meta.
        return {}, content
    end
    local decode = get_decoder()
    if not decode then
        return nil, nil, "frontmatter.parse: no JSON decoder available"
    end
    local meta, derr = decode(json_str)
    if not meta then
        return nil, nil, "frontmatter.parse: " .. tostring(derr)
    end
    return meta, body
end

return M