nvim/lua/git/show.lua

local diff = require("git.diff")
local log = require("log")
local repo = require("git.repo")
local util = require("util")

local M = {}

---@class ow.Git.DiffSection
---@field pre_path string   path on the parent side (`a/...`)
---@field post_path string  path on the current side (`b/...`)
---@field pre_blob string?
---@field post_blob string?

---@class ow.Git.ShowContext
---@field worktree string
---@field ref string         resolved commit SHA of the gitobject buffer
---@field parent_ref string? resolved parent commit SHA, nil for root commits

---@return ow.Git.ShowContext?
local function context()
    local worktree = vim.b.git_worktree
    local ref = vim.b.git_ref
    if not worktree or not ref then
        return nil
    end
    return { worktree = worktree, ref = ref, parent_ref = vim.b.git_parent_ref }
end

---Walk upward from the cursor to the enclosing `diff --git` line and parse
---the section's pre/post paths plus the pre/post blob SHAs from the `index`
---line.
---@param cursor_lnum integer  1-indexed
---@return ow.Git.DiffSection?
local function diff_section(cursor_lnum)
    local lines = vim.api.nvim_buf_get_lines(0, 0, cursor_lnum, false)
    local diff_lnum, diff_line
    for i = #lines, 1, -1 do
        if lines[i]:match("^diff %-%-git ") then
            diff_lnum = i
            diff_line = lines[i]
            break
        end
    end
    if not diff_lnum or not diff_line then
        return nil
    end
    local pre_path, post_path = diff_line:match("^diff %-%-git a/(.-) b/(.+)$")
    if not pre_path or not post_path then
        return nil
    end

    local header =
        vim.api.nvim_buf_get_lines(0, diff_lnum, diff_lnum + 20, false)
    local pre_blob, post_blob
    for _, l in ipairs(header) do
        if l:sub(1, 3) == "@@ " or l:match("^diff %-%-git ") then
            break
        end
        local pre, post = l:match("^index (%x+)%.%.(%x+)")
        if pre then
            pre_blob = pre
            post_blob = post
            break
        end
    end
    return {
        pre_path = pre_path,
        post_path = post_path,
        pre_blob = pre_blob,
        post_blob = post_blob,
    }
end

---@param sha string?
---@return boolean
local function is_zero(sha)
    return sha == nil or sha:match("^0+$") ~= nil
end

---Build a buffer holding the file's content at a given blob, named after the
---commit ref it corresponds to (so the name lines up with `git log` output
---instead of an opaque blob hash).
---@param worktree string
---@param blob string?
---@param path string
---@param ref string  the commit ref the blob represents (e.g. `<sha>` or `<sha>^`)
---@return integer
local function blob_buf(worktree, blob, path, ref)
    if is_zero(blob) then
        return diff.empty_buf({
            name = "git://" .. ref .. "/" .. path,
            bufhidden = "hide",
        })
    end
    ---@cast blob string
    local buf = diff.git_show_blob(worktree, blob)
    diff.set_buf_name_and_filetype(buf, "git://" .. ref .. "/" .. path)
    return buf
end

---@param worktree string
---@param blob string?
---@param path string
---@param ref string
local function show_blob(worktree, blob, path, ref)
    local buf = blob_buf(worktree, blob, path, ref)
    vim.cmd("normal! m'")
    vim.cmd("buffer " .. buf)
end

---@param ctx ow.Git.ShowContext
---@param section ow.Git.DiffSection
local function show_diff(ctx, section)
    if not section.pre_blob or not section.post_blob then
        log.warning("no index line; cannot determine blob SHAs")
        return
    end
    local parent = ctx.parent_ref or "0"
    local left =
        blob_buf(ctx.worktree, section.pre_blob, section.pre_path, parent)
    local right =
        blob_buf(ctx.worktree, section.post_blob, section.post_path, ctx.ref)
    vim.cmd("normal! m'")
    vim.cmd("buffer " .. left)
    vim.cmd("diffthis")
    vim.cmd("rightbelow vertical sbuffer " .. right)
    vim.cmd("diffthis")
    vim.cmd("wincmd p")
end

---@param worktree string
---@param ref string
function M.open_commit(worktree, ref)
    local result = vim.system(
        { "git", "show", ref },
        { cwd = worktree, text = true }
    )
        :wait()
    if result.code ~= 0 then
        log.error("git show %s failed: %s", ref, result.stderr or "")
        return
    end
    local lines = util.split_lines(result.stdout or "")
    local sha = repo.rev_parse(worktree, ref, true) or ref
    local parent = repo.rev_parse(worktree, ref .. "^", true)
    local buf = vim.api.nvim_create_buf(false, true)
    vim.bo[buf].buftype = "nofile"
    vim.bo[buf].bufhidden = "hide"
    vim.bo[buf].swapfile = false
    vim.api.nvim_buf_set_lines(buf, 0, -1, false, lines)
    vim.bo[buf].modifiable = false
    vim.bo[buf].modified = false
    pcall(vim.api.nvim_buf_set_name, buf, "git://" .. sha .. "/")
    vim.b[buf].git_worktree = worktree
    vim.b[buf].git_ref = sha
    vim.b[buf].git_parent_ref = parent
    vim.bo[buf].filetype = "git"
    vim.cmd("normal! m'")
    vim.cmd("buffer " .. buf)
end

---@return boolean dispatched true if the cursor was on an actionable line
function M.open_at_cursor()
    local ctx = context()
    if not ctx then
        return false
    end
    local cursor_lnum = vim.api.nvim_win_get_cursor(0)[1]
    local section = diff_section(cursor_lnum)
    if not section then
        return false
    end
    local parent = ctx.parent_ref or "0"

    local line = vim.api.nvim_get_current_line()
    if line:match("^diff %-%-git ") then
        show_diff(ctx, section)
        return true
    end
    if line:match("^%-%-%- ") then
        show_blob(ctx.worktree, section.pre_blob, section.pre_path, parent)
        return true
    end
    if line:match("^%+%+%+ ") then
        show_blob(ctx.worktree, section.post_blob, section.post_path, ctx.ref)
        return true
    end
    local prefix = line:sub(1, 1)
    if prefix == "+" then
        show_blob(ctx.worktree, section.post_blob, section.post_path, ctx.ref)
        return true
    elseif prefix == "-" then
        show_blob(ctx.worktree, section.pre_blob, section.pre_path, parent)
        return true
    end
    return false
end

return M