Biorhythms and Day Of The Week

This program calculates and displays biorhythm charts for a given birth date and target month, plotting Physical (23-day), Emotional (28-day), and Intellectual (33-day) cycles on a 32×24 character screen grid. The user enters birth date and desired month, and the program computes the number of elapsed days to determine each cycle’s phase using sine functions scaled to row positions. Input validation checks for impossible month numbers, day values over 31, and target dates before the birth year. The chart uses FAST mode for calculation, then switches to SLOW for display, with optional COPY output to a ZX Printer and navigation keys to step forward by month or restart entirely. A POKE to address 16418 is used to control the printer buffer/display area.

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Program Analysis

Program Structure

The program is organized into a clear sequence of phases:

1. Data entry (lines 2–67): Prompts for birth date, target month/year, printer choice, and a 4-letter name.
2. Leap-year detection (lines 77–165): Computes the number of days remaining in the birth year after the birth date (DA1).
3. Elapsed-year accumulation (lines 250–315): Counts days between end of birth year and start of target year (DA2), correctly inserting 366-day years for leap years.
4. Days-into-target-month (lines 320–404): Adds the cumulative day-of-year for the target month’s start (DA3).
5. Phase calculation (lines 405–424): Computes DAYT (total elapsed days) and derives display row offsets FUDGP, FUDGE, FUDGI for each cycle’s first day in the target month.
6. Array filling (lines 745–946): Runs in FAST mode, filling 32-element arrays P(), E(), I() with screen row values (0–20) for each day.
7. Display (lines 690–727, 952–967): Renders the chart frame, axis labels, month name, and plots P, E, I characters at computed positions.
8. Navigation (lines 990–1066): Allows advancing to the next month, restarting, or stopping.

Day-Count Arithmetic

The total elapsed days from birth to the start of the target month is built from three components:

• DA1: days remaining in the birth year after the birth date (lines 80–135 for non-leap, 155–160 for leap January/February).
• DA2: sum of days in all complete years between birth year+1 and the target year (lines 260–315), with leap years yielding 366.
• DA3: cumulative day offset to the start of the target month within the target year (lines 325–380), with a +1 correction at line 395 if the target year is a leap year and the month is March or later.

The combined total DAYT = DA1 + DA2 + DA3 - 1 (line 405) gives the number of days from the birth date to the first day of the target month.

Biorhythm Phase and Screen Mapping

Lines 422–424 compute the display column offset for day 1 of the target month within each cycle:

• FUDGP (Physical, period 23): INT(.5 + 22 - (DAYT/23 - INT(DAYT/23))*23)
• FUDGE (Emotional, period 28): INT(.5 + 27 - (DAYT/28 - INT(DAYT/28))*28)
• FUDGI (Intellectual, period 33): INT(.5 + 32 - (DAYT/33 - INT(DAYT/33))*33)

The fractional part of DAYT/period gives the phase offset, which is subtracted from the period length and rounded to find how many days remain in the current cycle at the month boundary. The sine functions in the array-fill loops then use this offset to place the plotted character at the correct screen row.

Array Filling and Sine Scaling

The array-fill loops (e.g., lines 755–805 for Physical) use two nested strategies:

• Forward fill: for day index P, compute column X = FUDGP + P; if X ≤ 31, set P(X+1) = 11 - 5*SIN(P/11.5*PI) (descending half of cycle).
• Backward fill: if X > 31, iterate backwards from the fudge offset to fill the ascending half using P(XX+1) = 11 + 5*SIN(Q/11.5*PI).

The centre row is 11 (midpoint of the 0–20 usable display range). Amplitude scaling is ±5 rows for Physical, ±7 for Emotional, ±9 for Intellectual, matching the relative importance traditionally assigned to each rhythm.

Notable Techniques

• FAST/SLOW switching: FAST (line 76, 745) is used during calculation to suppress screen updates and speed up the sine-heavy array fills; SLOW (line 690) is restored for display.
• POKE 16418: Used at line 406 (set to 0) and line 968 (set to 12). Address 16418 is the system variable DF_SZ, the number of lines in the lower display area. Setting it to 0 suppresses the lower screen during calculation; restoring it to 12 after plotting is unusual — the normal value is 2 — and may be an attempt to extend the display or manage COPY output.
• COPY for printer output: Line 970 conditionally issues COPY based on the user’s initial A$="Y" choice, sending the screen to a ZX Printer.
• Looping navigation: Lines 1061–1066 increment BMN, handle year rollover, and jump back to line 76 (FAST mode re-entry) to recalculate for the next month without re-entering the birth date.
• Array variable name collision: The program uses P both as a scalar loop variable (lines 755, 952) and as an array P() (line 750). BASIC resolves these as distinct entities, but the practice is potentially confusing.

Bugs and Anomalies

• DA validation is incomplete: The check at line 57 only rejects DA > 31; days 29–31 are accepted for February, and day 31 is accepted for 30-day months. This can produce slightly incorrect day counts.
• BYR=YR check (line 255): The program stops if the birth year equals the target year (line 257), but allows target years only one year ahead of birth without issue. However, the user could enter a target month/year in the birth year but a later month — this would produce incorrect results rather than being caught.
• 32-element vs 31-day arrays: Arrays are dimensioned to 32 (line 750–752) and the fill loop runs to 32, but only 31 elements are ever plotted. This is harmless but wastes a small amount of memory.
• Leap-year detection uses two-digit year: LY = (YR/4) - INT(YR/4) works correctly for years divisible by 4 within the two-digit range, but would misidentify year 00 (2000) as a leap year coincidentally correctly, and year 00 (1900) — also correctly, since 1900 was not a leap year but the program has no century handling.
• Month name PRINT statements (lines 701–712): Several month names are missing a comma before the closing quote (e.g., "FEBRUARY""" vs "JANUARY,"""), leading to inconsistent formatting in the display header.

Variable Summary

Variable	Purpose
A$	Printer flag (“Y”/”N”)
MN, DA, YR	Birth month, day, two-digit year
BMN, BYR	Target month and year for display
N$	User’s name (4 letters)
DA1	Days remaining in birth year after birth date
DA2	Days in complete years between birth and target
DA3	Day-of-year offset for start of target month
DAYT	Total elapsed days from birth to target month
FUDGP/FUDGE/FUDGI	Phase column offsets for P/E/I cycles
P(), E(), I()	Screen row arrays for each cycle’s 31 days
LY, LYY	Leap-year fraction test and next leap year