FUNCTION LoadBGIFont (filename AS STRING)
    LoadBGIFont = __BGI_Internal(1, 0, 0, 0, 0, filename, 0)
END FUNCTION

SUB UnloadBGIFont (fontHandle AS INTEGER)
    DIM discard AS LONG
    discard = __BGI_Internal(2, fontHandle, 0, 0, 0, "", 0)
END SUB

SUB DisplayBGIText (fontHandle AS INTEGER, startX AS SINGLE, startY AS SINGLE, text AS STRING, fontSize AS SINGLE, col AS LONG)
    DIM discard AS LONG
    discard = __BGI_Internal(3, fontHandle, startX, startY, fontSize, text, col)
END SUB

FUNCTION __BGI_Internal (mode AS INTEGER, handle AS INTEGER, arg1 AS SINGLE, arg2 AS SINGLE, arg3 AS SINGLE, textData AS STRING, col AS LONG)
    DIM filename AS STRING
    DIM fNum AS INTEGER
    DIM slot AS INTEGER
    DIM i AS LONG
    DIM c AS INTEGER
    DIM d AS STRING
    DIM header_size_pos AS LONG
    DIM header_size AS INTEGER
    DIM stroke_header_start AS LONG
    DIM strokes_offset AS INTEGER
    DIM asc_temp AS INTEGER
    DIM base_temp AS INTEGER
    DIM desc_temp AS INTEGER
    DIM char_offsets_pos AS LONG
    DIM widths_pos AS LONG
    
    DIM startX AS SINGLE
    DIM startY AS SINGLE
    DIM fontSize AS SINGLE
    DIM text AS STRING
    DIM font_height AS INTEGER
    DIM scale AS SINGLE
    DIM x AS SINGLE
    DIM ch AS INTEGER
    DIM stroke_pos AS LONG
    DIM cur_x AS SINGLE
    DIM cur_y AS SINGLE
    DIM byte1 AS INTEGER
    DIM byte2 AS INTEGER
    DIM op1 AS INTEGER
    DIM op2 AS INTEGER
    DIM m AS INTEGER
    DIM dx_raw AS INTEGER
    DIM dx AS INTEGER
    DIM dy_raw AS INTEGER
    DIM dy AS INTEGER
    DIM new_x AS SINGLE
    DIM new_y AS SINGLE


    STATIC init AS INTEGER
    STATIC __bgi_fontdata() AS STRING
    STATIC __bgi_metrics() AS LONG

    CONST MAX_FONTS = 10
    CONST M_INUSE = 1
    CONST M_FIRSTCHAR = 2
    CONST M_NUMCHARS = 3
    CONST M_ASCENDER = 4
    CONST M_BASELINE = 5
    CONST M_DESCENDER = 6
    CONST M_STROKESSTART = 7
    CONST METRICS_SIZE = 521

    IF NOT init THEN
        REDIM __bgi_fontdata(1 TO MAX_FONTS) AS STRING
        REDIM __bgi_metrics(1 TO MAX_FONTS, 1 TO METRICS_SIZE) AS LONG
        init = -1
    END IF

    SELECT CASE mode
        CASE 1 ' --- LOAD FONT ---
            filename = textData
            IF _FILEEXISTS(filename) = 0 THEN
                PRINT "File not found: "; filename
                __BGI_Internal = 0
                EXIT FUNCTION
            END IF
            slot = 0
            FOR i = 1 TO MAX_FONTS
                IF __bgi_metrics(i, M_INUSE) = 0 THEN
                    slot = i
                    EXIT FOR
                END IF
            NEXT i

            IF slot = 0 THEN
                PRINT "BGI Font Error: Maximum loaded font limit reached."
                __BGI_Internal = 0
                EXIT FUNCTION
            END IF

            ' Extract stream data safely 
            fNum = FREEFILE
            OPEN filename FOR BINARY AS #fNum
            __bgi_fontdata(slot) = SPACE$(LOF(fNum))
            GET #fNum, , __bgi_fontdata(slot)
            CLOSE #fNum

            d = __bgi_fontdata(slot)

            ' Validate signature header identity
            IF MID$(d, 1, 2) <> "PK" OR ASC(MID$(d, 3, 1)) <> 8 OR ASC(MID$(d, 4, 1)) <> 8 OR MID$(d, 5, 4) <> "BGI " THEN
                PRINT "Not a valid Borland BGI .CHR font file: "; filename
                __bgi_fontdata(slot) = ""
                __BGI_Internal = 0
                EXIT FUNCTION
            END IF

            ' Track string end parsing marker
            i = 9
            WHILE i <= LEN(d) AND ASC(MID$(d, i, 1)) <> 26
                i = i + 1
            WEND
            IF i > LEN(d) THEN
                PRINT "Invalid font format: Missing description terminator."
                __bgi_fontdata(slot) = ""
                __BGI_Internal = 0
                EXIT FUNCTION
            END IF

            header_size_pos = i + 1
            header_size = CVI(MID$(d, header_size_pos, 2))
            stroke_header_start = header_size + 1

            IF ASC(MID$(d, stroke_header_start, 1)) <> ASC("+") THEN
                PRINT "Not a stroked font."
                __bgi_fontdata(slot) = ""
                __BGI_Internal = 0
                EXIT FUNCTION
            END IF

            __bgi_metrics(slot, M_NUMCHARS) = CVI(MID$(d, stroke_header_start + 1, 2))
            __bgi_metrics(slot, M_FIRSTCHAR) = ASC(MID$(d, stroke_header_start + 4, 1))
            strokes_offset = CVI(MID$(d, stroke_header_start + 5, 2))

            ' Signed Int8 layout mappings
            asc_temp = ASC(MID$(d, stroke_header_start + 8, 1))
            IF asc_temp > 127 THEN __bgi_metrics(slot, M_ASCENDER) = asc_temp - 256 ELSE __bgi_metrics(slot, M_ASCENDER) = asc_temp
            base_temp = ASC(MID$(d, stroke_header_start + 9, 1))
            IF base_temp > 127 THEN __bgi_metrics(slot, M_BASELINE) = base_temp - 256 ELSE __bgi_metrics(slot, M_BASELINE) = base_temp
            desc_temp = ASC(MID$(d, stroke_header_start + 10, 1))
            IF desc_temp > 127 THEN __bgi_metrics(slot, M_DESCENDER) = desc_temp - 256 ELSE __bgi_metrics(slot, M_DESCENDER) = desc_temp

            ' Assign vector stroke references
            char_offsets_pos = stroke_header_start + 16
            FOR c = 0 TO __bgi_metrics(slot, M_NUMCHARS) - 1
                __bgi_metrics(slot, 8 + __bgi_metrics(slot, M_FIRSTCHAR) + c) = CVI(MID$(d, char_offsets_pos + c * 2, 2))
            NEXT c

            ' Map character layout metrics width boundaries
            widths_pos = char_offsets_pos + __bgi_metrics(slot, M_NUMCHARS) * 2
            FOR c = 0 TO __bgi_metrics(slot, M_NUMCHARS) - 1
                __bgi_metrics(slot, 265 + __bgi_metrics(slot, M_FIRSTCHAR) + c) = ASC(MID$(d, widths_pos + c, 1))
            NEXT c

            __bgi_metrics(slot, M_STROKESSTART) = stroke_header_start + strokes_offset
            __bgi_metrics(slot, M_INUSE) = -1

            __BGI_Internal = slot

        CASE 2 ' --- UNLOAD FONT ---
            IF handle >= 1 AND handle <= MAX_FONTS THEN
                __bgi_metrics(handle, M_INUSE) = 0
                __bgi_fontdata(handle) = ""
            END IF

        CASE 3 ' --- DISPLAY TEXT ---
            IF handle < 1 OR handle > MAX_FONTS THEN EXIT FUNCTION
            IF __bgi_metrics(handle, M_INUSE) = 0 THEN EXIT FUNCTION

            startX = arg1
            startY = arg2
            fontSize = arg3
            text = textData

            font_height = __bgi_metrics(handle, M_ASCENDER) - __bgi_metrics(handle, M_DESCENDER)
            IF font_height = 0 THEN font_height = 1
            
            scale = fontSize / font_height
            x = startX

            FOR ch = 1 TO LEN(text)
                c = ASC(MID$(text, ch, 1))
                
                IF c < __bgi_metrics(handle, M_FIRSTCHAR) OR c >= __bgi_metrics(handle, M_FIRSTCHAR) + __bgi_metrics(handle, M_NUMCHARS) OR __bgi_metrics(handle, 8 + c) = 0 THEN
                    x = x + 8 * scale
                    _CONTINUE
                END IF

                stroke_pos = __bgi_metrics(handle, M_STROKESSTART) + __bgi_metrics(handle, 8 + c)
                cur_x = x
                cur_y = startY

                DO
                    byte1 = ASC(MID$(__bgi_fontdata(handle), stroke_pos, 1))
                    byte2 = ASC(MID$(__bgi_fontdata(handle), stroke_pos + 1, 1))
                    stroke_pos = stroke_pos + 2

                    op1 = (byte1 AND 128) \ 128
                    op2 = (byte2 AND 128) \ 128
                    m = op1 + op1 + op2

                    IF m = 0 THEN EXIT DO
                    IF m = 1 THEN _CONTINUE 

                    dx_raw = byte1 AND 127
                    dx = dx_raw
                    IF dx > 63 THEN dx = dx - 128

                    dy_raw = byte2 AND 127
                    dy = dy_raw
                    IF dy > 63 THEN dy = dy - 128
                    dy = -dy 

                    new_x = x + dx * scale
                    new_y = startY + dy * scale

                    IF m = 2 THEN
                        cur_x = new_x
                        cur_y = new_y
                    ELSEIF m = 3 THEN
                        LINE (cur_x, cur_y)-(new_x, new_y), col
                        cur_x = new_x
                        cur_y = new_y
                    END IF
                LOOP

                x = x + __bgi_metrics(handle, 265 + c) * scale
            NEXT ch
    END SELECT
END FUNCTION