-module(log).
-export([open/2, open_disk/2, open_disk/3,
         append/2, tip/1, replay/3, entries/1,
         segments/1]).

%% Per-actor activity log — the canonical record of everything an
%% actor has emitted, in chronological order. Per design §15.2 this
%% lives on disk as numbered segment files; v1 started with an
%% in-memory backend (Step 3a) so the API + seq-number machinery
%% could be locked down before on-disk persistence (Step 3b) and
%% segment rotation (Step 3c.a — this revision).
%%
%% On-disk layout:
%%   <BasePath>/<ActorId>-NNNNNN.log
%%
%% NNNNNN is a 6-digit zero-padded segment index (000000..999999) so
%% file:list_dir's alphabetical ordering coincides with numeric. Each
%% segment file is the concat of length-prefixed frames; each frame
%% is `<<Len:32/big>>` + `term_codec:encode(Activity)`.
%%
%% In-memory state (a property list):
%%   [{actor, ActorId},
%%    {base, BasePath},          %% binary | charlist
%%    {seq, NextSeq},            %% next seq the log will assign
%%    {entries, [Activity, ...]}, %% flat, append order, oldest first
%%    {persisted, true|false},   %% does append write through?
%%    {seg_size, MaxBytes},      %% rotate when active segment > this
%%    {seg_lens, [N0, N1, ...]}]  %% entry count per segment in order
%%
%% `seg_lens` is the sole bookkeeping needed to compute (a) which
%% segment any given seq lives in, and (b) which slice of `entries`
%% is the active segment's contents to rewrite on append. The last
%% element is the active segment's length.

%% In-memory only — atoms accepted as BasePath for back-compat with
%% Step 3a tests that just want the API surface.
open(ActorId, BasePath) ->
    {ok, [{actor, ActorId}, {base, BasePath},
          {seq, 0}, {entries, []},
          {persisted, false}]}.

%% Disk-backed; default segment size = effectively unlimited (no
%% rotation). Use open_disk/3 with {segment_size, N} to enable.
open_disk(ActorId, BasePath) ->
    open_disk(ActorId, BasePath, [{segment_size, 1073741824}]). %% 1 GiB

open_disk(ActorId, BasePath, Opts) ->
    SegSize = proplist_get(segment_size, Opts, 1073741824),
    case load_all_segments(ActorId, BasePath) of
        {ok, SegEntries} ->
            %% SegEntries :: [[Entry, ...]] in segment-index order
            %% (empty list when no segments exist on disk).
            Lens0 = [length(S) || S <- SegEntries],
            %% Always have at least one active segment, even if empty.
            Lens = case Lens0 of
                       [] -> [0];
                       _ -> Lens0
                   end,
            Flat = flatten_segs(SegEntries),
            State = [{actor, ActorId}, {base, BasePath},
                     {seq, length(Flat)},
                     {entries, Flat},
                     {persisted, true},
                     {seg_size, SegSize},
                     {seg_lens, Lens}],
            {ok, State};
        {error, _} = E ->
            E
    end.

append(LogState, Activity) ->
    Seq = field(seq, LogState),
    Entries = field(entries, LogState),
    case lookup(persisted, LogState) of
        true ->
            SegLens = field(seg_lens, LogState),
            SegSize = field(seg_size, LogState),
            {NewSegLens, ActiveIdx, ActiveEntries} =
                place_append(Entries, Activity, SegLens, SegSize),
            Path = segment_path(field(actor, LogState),
                                field(base, LogState),
                                ActiveIdx),
            ok = write_segment(Path, ActiveEntries),
            NewState = replace_field(seq, Seq + 1,
                       replace_field(entries, Entries ++ [Activity],
                       replace_field(seg_lens, NewSegLens, LogState))),
            {ok, NewState, Seq};
        _ ->
            NewState = replace_field(seq, Seq + 1,
                       replace_field(entries, Entries ++ [Activity],
                                     LogState)),
            {ok, NewState, Seq}
    end.

tip(LogState) ->
    field(seq, LogState).

replay(LogState, InitAcc, Fun) ->
    Entries = field(entries, LogState),
    replay_loop(Entries, 0, InitAcc, Fun).

entries(LogState) ->
    field(entries, LogState).

%% Debug accessor: returns the in-memory seg_lens (count per segment
%% in index order). Used by rotation tests to assert that rotation
%% happened.
segments(LogState) ->
    case lookup(seg_lens, LogState) of
        undefined -> [];
        L -> L
    end.

%% --- internals ---

replay_loop([], _, Acc, _) -> Acc;
replay_loop([Act | Rest], Seq, Acc, Fun) ->
    replay_loop(Rest, Seq + 1, Fun(Act, Seq, Acc), Fun).

%% place_append/4 decides whether the new Activity extends the current
%% active segment or opens a fresh one, returning the resulting
%% seg_lens, the active segment's index, and the active segment's
%% complete entry list (the slice that needs to be (re)written to
%% disk).
%%
%% Rotation rule: if the active segment already on disk is at or past
%% the size threshold (encoded_size(OldActive) >= SegSize) AND it
%% already holds at least one entry, the new Activity opens a new
%% segment. A single entry larger than the threshold therefore lives
%% on its own — we never recurse rotating a one-entry segment.
%%
%% This is decided BEFORE the append (looking at the pre-append size),
%% so each segment file is written exactly once per append cycle.
place_append(OldEntries, Activity, SegLens, SegSize) ->
    {Pre, Last} = split_last(SegLens),
    PreCount = sum(Pre),
    OldActive = drop(PreCount, OldEntries),
    OldActiveSize = encoded_size(OldActive),
    case (OldActiveSize >= SegSize) andalso (Last >= 1) of
        true ->
            %% Rotate: new entry starts a brand-new segment.
            NewSegLens = SegLens ++ [1],
            NewActiveIdx = length(SegLens),
            {NewSegLens, NewActiveIdx, [Activity]};
        false ->
            %% Stay: extend current active.
            NewSegLens = Pre ++ [Last + 1],
            NewActiveIdx = length(Pre),
            {NewSegLens, NewActiveIdx, OldActive ++ [Activity]}
    end.

split_last([X]) -> {[], X};
split_last([H | T]) ->
    {Tl, Last} = split_last(T),
    {[H | Tl], Last}.

sum(L) -> sum_(L, 0).
sum_([], A) -> A;
sum_([H | T], A) -> sum_(T, A + H).

drop(0, L) -> L;
drop(_, []) -> [];
drop(N, [_ | T]) -> drop(N - 1, T).

%% flatten_segs/1 — concat a list of segments (each itself a list of
%% entries) into a single flat list, preserving order. Used by
%% open_disk to assemble the on-disk activity history from per-
%% segment loads. Implemented locally because lists:append/1 isn't
%% registered in this port — only lists:append/2.
flatten_segs([]) -> [];
flatten_segs([Seg | Rest]) -> Seg ++ flatten_segs(Rest).

encoded_size(Entries) ->
    byte_size(list_to_binary(
        [frame(term_codec:encode(E)) || E <- Entries])).

%% Try to read every segment file under BasePath matching the actor.
%% Returns {ok, [[Entry, ...]]} where the outer list is in segment-
%% index order. Empty when no segments exist.
load_all_segments(ActorId, BasePath) ->
    %% list_dir returns {ok, [Binary]} of entry names in sorted order
    %% per fed-prims contract.
    BaseChars = base_chars(BasePath),
    case file:list_dir(BaseChars) of
        {ok, Names} ->
            %% Erlang string literals are NOT charlists in this port,
            %% so build prefix/suffix as explicit char-code lists.
            Prefix = atom_to_list(ActorId) ++ [$-],
            Suffix = [$., $l, $o, $g],
            Indices = collect_segment_indices(Names, Prefix, Suffix),
            read_segments_in_order(Indices, ActorId, BasePath, []);
        {error, enoent} ->
            {ok, []};
        {error, R} ->
            {error, {read, R}}
    end.

collect_segment_indices([], _, _) -> [];
collect_segment_indices([Name | Rest], Prefix, Suffix) ->
    case parse_segment_name(Name, Prefix, Suffix) of
        {ok, N} ->
            [N | collect_segment_indices(Rest, Prefix, Suffix)];
        not_ours ->
            collect_segment_indices(Rest, Prefix, Suffix)
    end.

parse_segment_name(NameBin, Prefix, Suffix) when is_binary(NameBin) ->
    parse_segment_name(binary_to_list(NameBin), Prefix, Suffix);
parse_segment_name(Name, Prefix, Suffix) ->
    case strip_prefix(Name, Prefix) of
        {ok, Rest} ->
            case strip_suffix(Rest, Suffix) of
                {ok, NumStr} ->
                    case is_all_digits(NumStr) of
                        true -> {ok, list_to_integer(NumStr)};
                        false -> not_ours
                    end;
                not_ours -> not_ours
            end;
        not_ours -> not_ours
    end.

strip_prefix(Str, []) -> {ok, Str};
strip_prefix([C | Rest], [P | PRest]) ->
    case C =:= P of
        true -> strip_prefix(Rest, PRest);
        false -> not_ours
    end;
strip_prefix(_, _) -> not_ours.

strip_suffix(Str, Suffix) ->
    SL = length(Str),
    XL = length(Suffix),
    case SL >= XL of
        true ->
            Head = take_n_pl(SL - XL, Str),
            Tail = drop(SL - XL, Str),
            case Tail =:= Suffix of
                true -> {ok, Head};
                false -> not_ours
            end;
        false -> not_ours
    end.

take_n_pl(0, _) -> [];
take_n_pl(_, []) -> [];
take_n_pl(N, [H | T]) -> [H | take_n_pl(N - 1, T)].

is_all_digits([]) -> false;
is_all_digits(Chars) -> all_digits(Chars).

all_digits([]) -> true;
all_digits([C | Rest]) when C >= $0, C =< $9 -> all_digits(Rest);
all_digits(_) -> false.

%% read_segments_in_order/4 — fed-prims sorts list_dir alphabetically;
%% with 6-digit zero-padded names that coincides with numeric order.
%% But we also accept legacy unpadded names, so sort by index to be
%% defensive.
read_segments_in_order(Indices, ActorId, BasePath, Acc) ->
    Sorted = isort(Indices),
    read_each(Sorted, ActorId, BasePath, Acc).

read_each([], _, _, Acc) ->
    {ok, lists:reverse(Acc)};
read_each([Idx | Rest], ActorId, BasePath, Acc) ->
    Path = segment_path(ActorId, BasePath, Idx),
    case try_read_segment(Path) of
        {ok, Entries} ->
            read_each(Rest, ActorId, BasePath, [Entries | Acc]);
        {error, _} = E -> E
    end.

%% Tiny insertion sort over a small list of integers.
isort([]) -> [];
isort([H | T]) -> insert(H, isort(T)).
insert(X, []) -> [X];
insert(X, [Y | Rest]) when X =< Y -> [X, Y | Rest];
insert(X, [Y | Rest]) -> [Y | insert(X, Rest)].

%% segment_path/3 — charlist path to the Idx'th segment file.
segment_path(ActorId, BasePath, Idx) ->
    base_chars(BasePath) ++ [$/] ++ atom_to_list(ActorId)
        ++ [$-] ++ pad_int(Idx, 6) ++ [$., $l, $o, $g].

base_chars(B) when is_binary(B) -> binary_to_list(B);
base_chars(L) when is_list(L) -> L.

%% Zero-pad an integer to Width digits as a charlist.
pad_int(N, Width) ->
    Cs = integer_to_list(N),
    pad_left(Cs, Width).

pad_left(Cs, Width) ->
    case length(Cs) >= Width of
        true -> Cs;
        false -> pad_left([$0 | Cs], Width)
    end.

write_segment(Path, Entries) ->
    Frames = [frame(term_codec:encode(E)) || E <- Entries],
    file:write_file(Path, list_to_binary(Frames)).

%% frame/1 — prepend 4-byte big-endian length to Payload.
frame(Payload) when is_binary(Payload) ->
    L = byte_size(Payload),
    B3 = (L div 16777216) rem 256,
    B2 = (L div 65536) rem 256,
    B1 = (L div 256) rem 256,
    B0 = L rem 256,
    [B3, B2, B1, B0, Payload].

try_read_segment(Path) ->
    case file:read_file(Path) of
        {ok, Bin} ->
            try {ok, decode_frames(binary_to_list(Bin), [])}
            catch
                throw:Reason -> {error, {corrupt, Reason}};
                error:Reason -> {error, {corrupt, Reason}}
            end;
        {error, enoent} ->
            {ok, []};
        {error, R} ->
            {error, {read, R}}
    end.

decode_frames([], Acc) ->
    lists:reverse(Acc);
decode_frames([B3, B2, B1, B0 | Rest], Acc) ->
    Len = B3 * 16777216 + B2 * 65536 + B1 * 256 + B0,
    {Payload, Rest2} = take_n(Len, Rest),
    case term_codec:decode(list_to_binary(Payload)) of
        {ok, Term, _} -> decode_frames(Rest2, [Term | Acc]);
        {error, R} -> throw({decode, R})
    end;
decode_frames(_, _) ->
    throw(truncated_header).

take_n(0, R) -> {[], R};
take_n(N, [H | T]) ->
    {Hs, Tl} = take_n(N - 1, T),
    {[H | Hs], Tl};
take_n(_, []) ->
    throw(truncated_body).

%% --- proplist helpers ---

field(K, [{K, V} | _]) -> V;
field(K, [_ | Rest]) -> field(K, Rest);
field(_, []) -> erlang:error(badkey).

lookup(K, [{K, V} | _]) -> V;
lookup(K, [_ | Rest]) -> lookup(K, Rest);
lookup(_, []) -> undefined.

replace_field(K, V, []) -> [{K, V}];
replace_field(K, V, [{K, _} | Rest]) -> [{K, V} | Rest];
replace_field(K, V, [P | Rest]) -> [P | replace_field(K, V, Rest)].

proplist_get(K, [{K, V} | _], _) -> V;
proplist_get(K, [_ | Rest], Default) -> proplist_get(K, Rest, Default);
proplist_get(_, [], Default) -> Default.