Bridge Builder

Bridge Builder is an action game in which the player drops sections onto a growing bridge by pressing the spacebar at the right moment, with a falling figure animation when a section is missed and a rescue helicopter sequence for second chances. The program uses custom UDG (User-Defined Graphics) characters throughout — stored separately as a CODE file and reloaded at startup — for the bridge pieces, figure, and helicopter sprites. A high-score table with up to ten entries uses joystick input to scroll through an on-screen character set for name entry. Skill level inverts the delay loop counter so that level 10 yields the smallest delay, making the car move fastest. A built-in demo mode (variable dm) auto-plays the game using RND-based spacebar simulation until the player presses space to take over.

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Program Analysis

Program Structure

The program is organized into several functional regions:

1. Lines 1–7: Initialization — dimension arrays, set border/paper, initialize names to “nobody”, then jump to the high-score/title screen at line 4000.
2. Lines 9–14: Game setup — prompt for skill level, play a startup tune, configure demo mode.
3. Lines 40–85: Draw the game screen — bridge base, river, towers, and set initial game variables.
4. Lines 100–140: Main game loop — animate a moving car/person across the screen, poll for spacebar.
5. Lines 200–280: Drop routine — handle bridge section placement logic, determine scoring zones.
6. Lines 300–399: Miss/fall sequence — animate falling figure, life-belt deduction, helicopter rescue (via GO SUB 900).
7. Lines 900–999: Helicopter subroutine — animates a helicopter flying in, rescuing the figure, and flying away.
8. Lines 1000–1100: Bridge-complete celebration — animate final run, add 50 bonus points one at a time, restart.
9. Lines 3000–3050: Drowning/game-over sequence with “GLUG!!” text and sound.
10. Lines 3100–3240: High-score entry and bubble-sort.
11. Lines 4000–4070: Title/high-score display with color-cycling and demo mode launch.
12. Lines 9998–9999: SAVE/LOAD for program and UDG data.

UDG Usage

The game makes extensive use of User-Defined Graphics. UDG data is saved as a separate CODE block (SAVE "udg" CODE USR "a",21*8) and reloaded at startup (LOAD "udg" CODE), which is a common technique to preserve graphics across resets. The escapes \a through \u appear throughout PRINT statements and represent bridge sections, the walking figure, helicopter parts, water, and other sprites. At least 21 UDGs (168 bytes) are defined.

Skill Level and Delay Loop

After reading the skill level k (1–10), line 14 converts it with LET k=11-k. This means a skill level of 10 becomes k=1 (one inner loop iteration, fastest) and skill level 1 becomes k=10 (ten iterations, slowest). The inner delay loop at lines 119–121 iterates k times per animation frame, neatly inverting the difficulty scale without any conditional branching.

Demo Mode

The flag variable dm enables a fully automatic demo. In demo mode, line 12 forces k=0 (no delay) and displays a “DEMO” banner. Line 122 uses RND>.1 to trigger the drop subroutine with 90% probability when the car reaches position n, simulating a mostly-successful player. Demo mode runs until the player presses space on the title screen (line 4040 polls INKEY$ inside a color-cycling loop). Game over in demo mode returns directly to the title screen (line 3080).

Joystick-Based Name Entry

The high-score name entry (lines 3135–3190) is a custom scrolling character picker. A 34-character string z$ is doubled to allow circular scrolling. STICK (1,1) returns 8 for right and 4 for left, used to increment or decrement the cursor position c. STICK (2,1)=1 fires to select a character, appending it to h$(10). CHR$ 13 (Enter) also terminates entry via line 3173. Bounds-checking at lines 3171–3172 wraps c within 1–34.

High-Score Bubble Sort

Lines 3200–3240 implement a standard bubble sort over the parallel arrays h() (scores) and h$() (names). A flag f is reset to 0 before each pass; any swap sets f=1, triggering another pass via IF f=1 THEN GO TO 3200. Both the numeric and string arrays are swapped in tandem using temporary variables t and a$.

Screen Drawing Techniques

The game uses AT, TAB, PAPER, INK, OVER, INVERSE, and FLASH inline within PRINT statements extensively to build a multi-colored game screen without clearing it between updates. The river (lines 50–70) is drawn with block graphics and UDGs using color attributes to create the illusion of depth. The bridge towers at lines 60 use TAB z and AT 15+z, 31-z to draw diagonal supports.

Score Display and Bonus Points

The score s is updated in line 137 with the formula INT((11-k)/2)+(k+10), giving higher reward for harder skill levels. The bridge-completion bonus at line 1030 adds 50 points one at a time in a loop, printing each updated score with FLASH 1 for visual effect.

Notable Techniques

• POKE 23693,56 sets the screen scroll delay counter to prevent automatic scrolling during printing.
• POKE 23624,141 sets the system variable controlling the INPUT line position.
• String slicing on a literal: "50 points for completing bridge"(z+1) at line 1010 scrolls the message across the screen one character at a time by advancing the slice start index.
• PAPER 8 and INK 9 are used in several places to preserve existing color attributes (transparent ink/paper), a useful technique for overlaying sprites without disturbing background colors.
• The "S" AND lv<>1 idiom at line 390 appends an “S” to “LIFE-BELT” only when the count is not 1, providing grammatically correct pluralization.
• Line 122 calls GO SUB 2 (demo auto-drop) and GO SUB 200 (the drop action) — GO SUB 200 without a RETURN causes execution to fall through to line 130 via the GO TO 130 at line 280, which is intentional flow control.

Anomalies and Notes

• Line 300 is referenced by GO TO 300 at line 140 but does not exist; execution falls through to line 320. This is the standard “fall to next line” technique.
• Variable lv is used in line 386 as lowercase but referenced as LV (uppercase) in the PRINT at line 390 ("S" AND LV<>1). In Sinclair BASIC, variable names are case-insensitive in execution, so this is not a bug.
• Line 82 uses FOR z=1 TO n-1 to print bridge sections, but after a miss at line 399 execution returns to line 82 directly, redrawing only sections already placed — correctly restoring the board without re-entering the full setup.
• The GO SUB 2 call in demo mode (line 122) targets line 2, which simply checks INKEY$=" " and returns — effectively a lightweight “did the player press space?” check usable as a subroutine.