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

# Color definitions

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

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

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

display() {
    # 1 - slide contents
    # 2 - slide name

    slide_contents="$1"

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

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

    # Move the cursor to the center.
    read -r LINES COLUMNS < <(stty -F /dev/tty size)
    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
        # Print the contents of the slide file, 
        # line by line.
        printf "%s" "$line"
        # Move down and back after each print.
        printf '\e[%sD\e[B' "${#line}"
    done <<< "$slide_contents"

}

main() {

    slides_dir="${1:-./}"
    slides=("$slides_dir"/[0-9]*.txt)
    i=0
    while true; do

        # Exit after last slide.
        [[ "$i" -gt "$((${#slides[@]} - 1))" ]] && {
            printf '\e[?25h'
            exit
        }

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

        # Navigate on j/k/n/p and quit on q.
        display "$(<${slides[$i]})" "${slides[$i]}"
        read -rsn1 input
        case "$input" in 
            "j"|"n")
                ((++i))
                ;;
            "k"|"p")
                ((--i))
                ;;
            "q")
                # Return the cursor on exit.
                printf '\e[?25h'
                exit
                ;;
        esac
    done

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

}

main "$@"