Crack Safe

This program simulates a combination lock safe-cracking game in which the player must guess three secret numbers between 1 and 50. The screen displays a graphical safe door drawn with PLOT, DRAW, and CIRCLE commands, including concentric filled circles for a dial effect using INK 0 and INVERSE. As the player dials each number, a BEEP sequence plays with pitch values calculated from a parabolic curve centered on the target number, providing an audio hint about proximity. The player starts with 10 security units, losing one for each incorrect guess; reaching zero triggers a flashing security alert and a 50-note alarm loop before halting.

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Program Analysis

Program Structure

The program is organized into three distinct phases: screen setup and graphics (lines 5–45), the main game loop across three combinations (lines 50–200), and the endgame resolution (lines 210–220). The outer FOR c=1 TO 3 loop at line 50 controls the three combination stages, with a shared input/validation/feedback cycle inside it.

Screen and Graphics Setup

Lines 5–8 paint a green background and white inner panel, then use PLOT/DRAW to draw a rectangular border at pixel coordinates (64,23) with dimensions 127×128. Lines 15–20 draw the safe dial: a solid ring of concentric circles in INK 1 (blue) for radii 25–45, then a smaller filled circle in INK 0 (black) for radii 56–64 to erase the center, giving a donut appearance. Line 40 draws solid block-graphic borders down the left and right edges using the \:: (█) escape character.

Combination and Pitch Calculation

At line 60, each secret number n is chosen randomly: INT(RND*46+5), giving a range of 5–50. Line 70 populates the 50-element array b() with pitch values using a piecewise linear formula centered on n:

• For i < n: pitch = 25 - (n-1)/2 (a constant below target, scaled)
• For i > n: pitch = 25 - (i-n)/2 (decreasing above target)
• For i = n: pitch is set to -10 (a distinct low note marking the exact target)

The Boolean arithmetic (i<n) and (i>n) evaluate to -1 (true) or 0 (false) in Spectrum BASIC, so the expression correctly selects the appropriate branch without an IF statement — a classic Sinclair idiom.

There is a subtle anomaly in the formula: the i < n branch evaluates to 25 - (n-1)/2 regardless of i, making all positions below the target play the same pitch. This means the audio hint is asymmetric: the pitch only changes as the dial passes above the target number.

Dial Animation

Lines 100–120 animate the dial movement. The loop FOR k=x TO d STEP 1-2*(d<x) sweeps from the current position x to the entered number d, stepping +1 or -1 depending on direction. Each step plays BEEP 0.1,b(k), giving an audible sweep across the pitch array. After the sweep, x is updated to d.

Input Validation

Line 90 rejects values outside 1–50 or equal to the current dial position (d=x), displaying a flashing “UNAUTHORIZED ENTRY” message and immediately setting su=0 to trigger the alarm. Line 80 uses INPUT FLASH 1; to present a flashing prompt. The condition d=x prevents re-entering the current number, simulating the requirement to move the dial.

Security Units and Endgame

The player starts with su=10 security units (line 10). Each wrong guess decrements su at line 160. A warning message appears at line 180 when only one unit remains. Reaching zero sends the program to line 210, where a 50-iteration double-beep alarm plays before STOP. Completing all three combinations sends control past line 200’s NEXT c to line 210, where the IF su=0 check is false, so execution falls through to the congratulatory message at line 220.

Notable Techniques

• Boolean expressions used as numeric values in arithmetic (lines 70, 110) — a hallmark Sinclair BASIC optimization avoiding IF branches inside tight loops.
• Two-digit display padding at line 125: if d < 10, a leading zero is printed manually by printing 0 then d at the same position.
• Ascending confirmatory beeps on successful unlock (line 140): FOR i=1 TO c: BEEP 0.5,-5*i plays c descending-pitch tones, distinguishing first, second, and third unlocks audibly.
• The array b(50) is declared with DIM b(50) at line 10 but indexed 1–50, which is correct for Spectrum BASIC (1-based arrays).

Potential Bugs and Anomalies

• The pitch formula for positions below n is constant (does not vary with i), so the audio feedback is one-sided. This may be intentional to increase difficulty, but it appears to be a mathematical oversight in the formula.
• If d = n on a correct guess, line 115’s NEXT k will attempt to continue the loop after line 110’s FOR k, which is fine structurally, but the b(n)=-10 pitch will play as part of the sweep — the distinctive “click” note occurs at the target number during the sweep itself, which is a nice side effect.
• The variable l used in lines 5 and 7 is reused as a loop variable, but since these loops complete before l is needed again, there is no conflict.