#!/usr/bin/env bash
# 
# Usage: shlide path/to/slides/
# Each slide is a textfile under path/to/slides

# Color definitions

BLK="\e[38;5;30m"
RED="\e[38;5;31m"
GRN="\e[38;5;32m"
YLW="\e[38;5;33m"
BLU="\e[38;5;34m"
PUR="\e[38;5;35m"
CYN="\e[38;5;36m"
RST="\e[0m"

lines() {
    mapfile -tn 0 lines < "$1"
    printf '%s\n' "${#lines[@]}"
}

longest_line() {
    max=0 
    local IFS=
    while read -r line; do
        l=$(ansi_filter $(colorify "$line"))
        if [ "${#l}" -gt "$max" ]; then max="${#l}"; fi
    done < "$1"
    printf '%s\n' "$max"
}

colorify() {
    # 'eval' hack to achieve substitution for colors.
    eval "declare dummy=\"$1\""
    printf '%b' "$dummy"
}

# Filter out color sequences.
shopt -s extglob
ansi_filter() {
    local IFS=
    echo "${1//$'\e'[\[(]*([0-9;])[@-n]/}"
    #" A little fix to prevent vim syntax highlighting from breaking.
}


display() {
    # 1 - slide contents
    # 2 - slide name
    # 3 - current slide nr.
    # 4 - total nr. of slides

    slide_contents="$1"

    # Hides the cursor.
    printf '\e[?25l'

    # Clear the screen.
    printf '\e[2J'

    # Get screen size
    shopt -s checkwinsize; (:;:)

    # Write slide number at bottom left
    printf '\e[H'
    printf '\e[%sB%s/%s' "$LINES" "$(($3+1))" "$4"

    # Custom calculations to center text
    height=$(lines "$2")
    width=$(longest_line "$2")

    # Rough estimates for the true center.
    ((l=$LINES/2 - $height/2))
    ((c=$COLUMNS/2 - $width/2))

    printf '\e[%s;%sH' "$l" "$c"

    while IFS= read -r line; do
        reduce=0
        # Print the contents of the slide file, 
        # line by line.
        l=$(colorify "$line")
        printf '%s' "$l"
        case $line in
            "" | *"\n"*)
                ((++reduce));;
        esac
        # Move down and back after each print.
        l=$(ansi_filter "$l")
        printf '\e[%sD\e[B' "$((${#l} - $reduce))"
    done <<< "$slide_contents"

}

die() {
    printf '\e[2J'
    printf '\e[?25h'
    exit 0
}

display_end() {
    read -r LINES COLUMNS < <(stty -F /dev/tty size)
    ((l=$LINES/2))
    ((c=$COLUMNS/2 - 8))
    printf '\e[2J'
    printf '\e[0;%sH' "$c"
    printf 'END. Press q to quit.'
}

main() {

    slides_dir="${1:-./}"
    slides=("$slides_dir"/[0-9]*.txt)
    i=0
    while true; do
        # Clear the screen.
        printf '\e[2J'

        # Capture Ctrl+C.
        trap 'die' INT

        # Display END reached prompt, and then exit
        [[ "$i" -eq "${#slides[@]}" ]] && {
            display_end
            read -rsn1 input
            case "$input" in
                "j"|"n"|"q")
                    die
                    ;;
                *)
                    ((--i))
                    ;;
            esac
        }

        # Don't go below 0.
        [[ "$i" -lt 0 ]] && i=0

        # Navigate on j/k/n/p and quit on q.
        display "$(<${slides[$i]})" "${slides[$i]}" "$i" "${#slides[@]}"
        read -rsn1 input
        case "$input" in 
            "j"|"n")
                ((++i))
                ;;
            "k"|"p")
                ((--i))
                ;;
            "q")
                # Return the cursor on exit.
                die
                ;;
        esac
    done

    # Return the cursor.
    printf '\e[?25h'

}

main "$@"