#!/bin/bash
# Usage: get_ytpljson.sh arg1 arg2
# Input: posargs: arg1: YouTube playlist ID
#                 arg2: Google API key
# Output: file: JSON file
# Version: 0.0.1

max_api_calls="100";

yell() { echo "$0: $*" >&2; } # print script path and all args to stderr
die() { yell "$*"; exit 111; } # same as yell() but non-zero exit status
must() { "$@" || die "cannot $*"; } # runs args as command, reports args if command fails
get_response() {
    # Input: arg1: YouTube playlist ID
    #        arg2: Google API key
    #        arg3: pageToken (optional)
    # Output: stdout: JSON response from googleapis.com
    # Depends: curl 7.81.0
    #          BK-2020-03: die()
    
    local PLAYLIST_ID API_KEY PAGE_TOKEN URL;

    # Set the playlist ID and API key
    PLAYLIST_ID="$1";
    API_KEY="$2";
    PAGE_TOKEN="$3";

    # Check inputs
    if [[ $# -lt 2 ]]; then die "FATAL:Incorrect arg count:$#"; fi;
    
    # Base URL
    URL="https://www.googleapis.com/youtube/v3/playlistItems?part=snippet";

    # Append playlist ID
    URL="$URL""&playlistId=""$PLAYLIST_ID";

    # Append API key
    URL="$URL""&key=""$API_KEY";

    # Append page token if it exists
    if [[ -n "$PAGE_TOKEN" ]]; then
        URL="$URL""&pageToken=""$PAGE_TOKEN";
    fi;
    
    curl -s "$URL";
    #curl -s "https://www.googleapis.com/youtube/v3/playlistItems?part=snippet&playlistId=$PLAYLIST_ID&key=$API_KEY"; # example
    
}; # Stdout: JSON from YouTube v3 API
check_next_page() {
    # Input: arg1: json string
    # Depends: jq 1.6

    # Checks if key ".nextPageToken" present
    if jq -e '.nextPageToken' < <(printf "%s" "$1") 1>/dev/random 2>&1; then
        return 0;
    else
        return 1;
    fi;

}; # returns true if '.nextPageToken' present
get_next_page() {
    # Input: arg1: json string containing the key 'nextPageToken'
    # Output: stdout: the value of the first 'nextPageToken' key
    #         exit code: 0: key '.nextPageToken' detected
    #                    1: key '.nextPageToken' not detected
    # Depends: jq 1.6
    local output;
    if [[ $# -ne 1 ]]; then die "Incorrect arg count:$#"; fi;

    if jq -e '.nextPageToken' < <(printf "%s" "$1") 1>/dev/random 2>&1; then
        output="$(jq -r '.nextPageToken' < <(printf "%s" "$1") | head -n1)";
        printf "%s" "$output";
        return 0;
    else
        return 1;
    fi;
}; # stdout: value from key "pageToken"

main() {
    # Depends: bash 5.1.16, GNU Coreutils 8.32 (date)
    #          BK-2020-03: yell()
    # Ref/Attrib: [1]: "Obtaining authorization credentials" https://developers.google.com/youtube/registering_an_application
    #             [2]: "Implementation: Playlists" https://developers.google.com/youtube/v3/guides/implementation/playlists
    #             [3]: "Implementation: Pagination", https://developers.google.com/youtube/v3/guides/implementation/pagination
    
    local n out_path;
    declare -a out_list;
    
    # Check input
    if [[ $# -ne 2 ]]; then die "FATAL:Incorrect number of args:$#"; fi;

    # Set the playlist ID and API key
    playlistId="$1"; # See ref [2]
    apiKey="$2"; # See ref [1]

    # Set dynamic variables according to environment
    out_dir="$(pwd)"; # output to present working directory
    out_filename="$(date +%Y%m%dT%H%M%S%z)"_"$playlistId"..playlist_items.json;
    out_path="$out_dir"/"$out_filename";
    
    # Make initial curl request to the YouTube Data API
    response="$(get_response "$playlistId" "$apiKey")";

    # # debug
    # if check_next_page "$response"; then
    #     yell "DEBUG:nextPageToken detected";
    # fi;

    # Make follow-up requests. See ref [3]
    n=0;
    while check_next_page "$response"; do        
        # Get page token from response
        pageToken="$(get_next_page "$response")";
        # Update response
        response="$(get_response "$playlistId" "$apiKey" "$pageToken")";
        # Record response
        out_list+=("$response");
        
        # Sanity check
        if [[ $n -gt $max_api_calls ]]; then die "FATAL:Too many API calls:$n"; fi;
        ((n++));
    done;

    # Write results
    printf "%s\n" "${out_list[@]}" > "$out_path";

    # Print stats
    yell "STATUS:Performed $n API calls."
    out_lc="$(printf "%s\n" "${out_list[@]}" | wc -l)";
    yell "STATUS:Wrote $out_lc lines to $out_path";
    
    # Use jq to extract the publishedAt field for each playlist item
    #PUBLISHED_AT=$(echo "$response" | jq -r '.items[].snippet.publishedAt')

}; # main program

main "$@";

# Author: Steven Baltakatei Sandoval
# License: GPLv3+