从'计算两个日期天数之差'程序介绍RPG语言
RPG，这个不是
Role Playing Game
的缩写，而是一种编程语言，Report Program Generator的缩写，是一种起源于用在DEC及IBM的小型机操作系统中编制报表程序的编程语言，一种运行在IBM的AS400小型机上的高级语言，一种完全过程化的程序语言。下面的这段代码是用来计算两个日期之差的天数：
H*
H* Author : KXT
H*
D*
D TOTAL_DAYS S 8S 0
D BEGIN_REMAIN S 3S 0
D END_PASS S 3S 0
D MONTH_DAYS S 2S 0 DIM(12) CTDATA PERRCD(12)
D*
D BEGIN_DATE S 8S 0
D BEGIN_YEAR S 4S 0
D BEGIN_MONTH S 2S 0
D BEGIN_DAY S 2S 0
D END_DATE S 8S 0
D END_YEAR S 4S 0
D END_MONTH S 2S 0
D END_DAY S 2S 0
D TEMP_DATE S 8S 0
D TEMP_YEAR S 4S 0
D*
D REM_DATA1 S 4S 0
D REM_DATA2 S 4S 0
D TEMP_DATA S 4S 0
D I S 4S 0
D*
D IS_LEAP S LIKE(*IN99)
D NEED_TRANSFER S LIKE(*IN99)
D*
C*
C* Main Logic
C EXSR SR999
C EXSR SR001
C EXSR SR998
C*
C* Processor
C SR001 BEGSR
C BEGIN_DATE IFGT END_DATE
C MOVE *ON NEED_TRANSFER
C MOVE BEGIN_DATE TEMP_DATE
C MOVE END_DATE BEGIN_DATE
C MOVE TEMP_DATE END_DATE
C ENDIF
C* Get begin year, month, day
C EVAL BEGIN_YEAR = %DIV(BEGIN_DATE:10000)
C EVAL TEMP_DATA = %REM(BEGIN_DATE:10000)
C EVAL BEGIN_MONTH = %DIV(TEMP_DATA:100)
C EVAL BEGIN_DAY = %REM(TEMP_DATA:100)
C* Get end year, month, day
C EVAL END_YEAR = %DIV(END_DATE:10000)
C EVAL TEMP_DATA = %REM(END_DATE:10000)
C EVAL END_MONTH = %DIV(TEMP_DATA:100)
C EVAL END_DAY = %REM(TEMP_DATA:100)
C* If begin and end is same year
C BEGIN_YEAR IFEQ END_YEAR
C EVAL I = BEGIN_MONTH
C MOVE *ZERO TEMP_DATA
C EVAL TEMP_DATA = END_MONTH
C DOW I < TEMP_DATA
C ADD MONTH_DAYS(I) TOTAL_DAYS
C MOVE BEGIN_YEAR TEMP_YEAR
C EXSR SR002
C IS_LEAP IFEQ *ON
C I ANDEQ 2
C EVAL TOTAL_DAYS = TOTAL_DAYS + 1
C ENDIF
C EVAL I = I + 1
C ENDDO
C EVAL TOTAL_DAYS = TOTAL_DAYS + END_DAY - BEGIN_DAY
C* If both year not equal
C ELSE
C* Calculate begin date remaining days
C EVAL I = BEGIN_MONTH
C DOW I <= 12
C ADD MONTH_DAYS(I) BEGIN_REMAIN
C MOVE BEGIN_YEAR TEMP_YEAR
C EXSR SR002
C IS_LEAP IFEQ *ON
C I ANDEQ 2
C EVAL BEGIN_REMAIN = BEGIN_REMAIN + 1
C ENDIF
C EVAL I = I + 1
C ENDDO
C EVAL BEGIN_REMAIN = BEGIN_REMAIN - BEGIN_DAY
C* Calculate end date passing days
C EVAL I = 1
C DOW I < END_MONTH
C ADD MONTH_DAYS(I) END_PASS
C MOVE END_YEAR TEMP_YEAR
C EXSR SR002
C IS_LEAP IFEQ *ON
C I ANDEQ 2
C EVAL END_PASS = END_PASS + 1
C ENDIF
C EVAL I = I + 1
C ENDDO
C EVAL END_PASS = END_PASS + END_DAY
C* Calculate other year total days
C EVAL I = BEGIN_YEAR + 1
C DOW I < END_YEAR
C ADD 365 TOTAL_DAYS
C MOVE I TEMP_YEAR
C EXSR SR002
C IS_LEAP IFEQ *ON
C EVAL TOTAL_DAYS = TOTAL_DAYS + 1
C ENDIF
C EVAL I = I + 1
C ENDDO
C* Calculate total diff days
C EVAL TOTAL_DAYS = TOTAL_DAYS + BEGIN_REMAIN
C EVAL TOTAL_DAYS = TOTAL_DAYS + END_PASS
C ENDIF
C NEED_TRANSFER IFEQ *ON
C EVAL TOTAL_DAYS = TOTAL_DAYS * -1
C ENDIF
C TOTAL_DAYS DSPLY
C ENDSR
C*
C* Check if leap year
C SR002 BEGSR
C EVAL REM_DATA1 = %REM(TEMP_YEAR:400)
C REM_DATA1 IFEQ *ZERO
C MOVE *ON IS_LEAP
C ELSE
C EVAL REM_DATA1 = %REM(TEMP_YEAR:4)
C EVAL REM_DATA2 = %REM(TEMP_YEAR:100)
C REM_DATA1 IFEQ *ZERO
C REM_DATA2 ANDNE *ZERO
C MOVE *ON IS_LEAP
C ELSE
C MOVE *OFF IS_LEAP
C ENDIF
C ENDIF
C ENDSR
C*
C* Finalization
C SR998 BEGSR
C MOVE *ON *INLR
C RETURN
C ENDSR
C*
C* Initialization
C SR999 BEGSR
C *ENTRY PLIST
C PARM BEGIN_DATE
C PARM END_DATE
C*
C MOVE *OFF IS_LEAP
C MOVE *OFF NEED_TRANSFER
C MOVE *ZERO TOTAL_DAYS
C MOVE *ZERO BEGIN_REMAIN
C MOVE *ZERO END_PASS
C*
C ENDSR
C*
**CTDATA MONTH_DAYS
312831303130313130313031
每一行最前面的字母都有特别的意思。。
H(Header的缩写，这个是我自己认为的，以下皆同) - 可以在这里写些代码的描述；
F(File) - 数据库文件的定义
D(Definition) - 变量的定义
C(Calculation) - 主程序
I(Input) - 输入
O(Output) - 输出
每一行加*的都代表那一行是注释。
我们是在IBM的AS400机器上写RPG的，界面是黑底绿字，字体颜色可以修改的，RPG程序代码的位置都是写死的，跟汇编语言是一样的，语法其实也是很像汇编的。
据我了解，貌似国内就只有银行，保险和证券是用RPG语言的，其他好像很少有公司用到这门语言，其实RPG历史还是蛮悠久的，用着用着还是蛮好玩的。
维基百科 ： http://en.wikipedia.org/wiki/IBM_RPG