Biorhythm

This program calculates and displays biorhythm charts for three cycles — Physical (23 days), Emotional (28 days), and Intellectual (33 days) — based on a user-supplied date of birth and today’s date. Date validation at line 1000 checks string length, delimiter placement, and digit validity before converting the DD.MM.YY input into a day-count using month-length data encoded in the string `m$` as ASCII offset values (e.g., `CODE m$(m)-20` yields the number of days in month `m`). Leap year correction is applied when the year is divisible by 4 and the accumulated day count has passed February. The chart is plotted using PLOT and SIN with a scaling factor of 20 pixels amplitude, and month boundaries are marked by vertical tick lines driven by the same `m$` encoding used during date parsing.

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Program Structure

The program is divided into four logical sections:

1. Lines 10–90: Setup, color attributes, and input loop for birth date and today’s date.
2. Lines 100–340: Cycle calculation and chart rendering including sine-wave plotting and month-boundary markers.
3. Lines 1000–1550: Subroutine that validates a date string and converts it to an absolute day number stored in n.
4. Line 2000–2010: Error handler that sets flag x=1 and prints a flashing “INVALID DATE” message.

The m$ Encoding Trick

The string m$="303232332323" at line 20 encodes month lengths in a compact form. Each character’s ASCII code minus 20 yields the number of days in that month: '3' = 51−20 = 31, '0' = 48−20 = 28, '2' = 50−20 = 30. This avoids a DATA statement or array, and the same string is reused both in the date-to-day conversion (lines 1090–1530) and in the month-boundary tick-mark loop (lines 300–330).

Date Validation Subroutine (Lines 1000–1550)

The subroutine performs several layered checks before attempting numeric conversion:

• Exact length of 8 characters (line 1010).
• Dots at positions 3 and 6 (line 1020).
• Digit characters at positions 1–2, 4–5, and 7–8 via a loop stepping by 3 (lines 1030–1050).
• Non-zero day, month, and year, and month ≤ 12 (line 1070).

An anomaly exists at line 1090: it calculates the days in month m into n (including leap-day adjustment for February), but this value is immediately overwritten by LET n=0 at line 1500 without being used. The intent was likely to validate that the day number does not exceed the month length, but that check was never implemented — so invalid days such as 31 in April pass silently.

Day-Count Calculation

Lines 1500–1550 sum month lengths for all months before m, add a leap-year correction if the accumulated count has passed day 59 (end of February), then add the day-of-month and an approximation of prior complete years using INT (365.25*(y-1)). This gives a relative Julian-style day number sufficient for computing the difference between two dates. The two-digit year y means dates in different centuries are not distinguished, though this is typical for the era.

Biorhythm Calculation and Plotting

Line 130 derives cycle periods: b(1)=23, b(2)=28, b(3)=33, matching Physical, Emotional, and Intellectual cycles. Line 140 computes c(j), the phase offset (days into the current cycle). The sine wave is plotted at line 230:

PLOT INK j; k, SIN((k + 2*c(j)) * PI / b(j)) * 20 + ((4-j)*48-28)

The factor 2*c(j) in the argument shifts the curve horizontally so the chart’s left edge represents the current date. Each cycle is drawn in a different INK color (1=blue, 2=red, 3=green) and centered on its own horizontal baseline at vertical positions 20, 68, and 116 pixels respectively.

Month Boundary Markers (Lines 300–330)

A second pass over all 256 x-coordinates draws short vertical tick marks at month boundaries. Variable d accumulates 0.5 per pixel column and is compared against CODE m$(m)-20 (days in month m). When the threshold is exceeded, d resets and m advances. Note that d is also used as the day-of-month variable in the date parsing subroutine (line 1060), so if the chart section is reached, d retains whatever value was left from parsing. This is benign because d is effectively initialized to a small value before the comparison matters, but it is a variable reuse that could cause confusion.

Notable Techniques and Anomalies

• The typo “Pleasr” (missing ‘e’) in the PRINT statement at line 30 is a cosmetic bug in the original listing.
• Error handling uses a flag variable x rather than structured error trapping, with GO TO 2000 used as a shared error exit from multiple validation points.
• The DIM n$(3,12) at line 100 allocates fixed-length strings; “Physical” (8 chars), “Emotional” (9), and “Intellectual” (12) are padded to 12 by the DIM, so labels display correctly without manual padding.
• Line 1090 computes a value into n that is immediately discarded by line 1500’s LET n=0, representing an incomplete validation or refactoring artifact.