I'm very new to C programming, but trying to understand the part of OpenGD77 code that handle the display of DMRID database data.
The most complicated part of the function, and I can't wrap my head around it is the part where the data is more than 1 line (more than 21 character).
I can understand the first few conditions,
if there is no data then do nothing.
if the data length is less than 16 character, then display it on one line with big font.
if the data is more than 2 line (more than 42 character) then make it 42 character and treat it as 2 line
if the data is more than 1 line (meaning it is 2 line because we have just eliminated anything more than 2 line). now I'm confused.
I think I don't understand this line
char *p = buffer + 20;
What is it? how to add a string with number? there has to be something more than I can grasp.
I would be appreciated if someone can explain this to me, or point me to a link, so I can read.
Is it a pointer that point to the position of buffer[21]?
a full code of the function below
Code: Select all
static void displaySplitOrSpanText(uint8_t y, char *text)
{
if (text != NULL)
{
uint8_t len = strlen(text);
if (len == 0) // no text, doing nothing
{
return;
}
else if (len <= 16) // less than 16 characters, display 1 line big font
{
displayPrintCentered(y, text, FONT_SIZE_3);
}
else // more than 16 characters
{
uint8_t nLines = len / 21 + (((len % 21) != 0) ? 1 : 0);
if (nLines > 2) // more than 2 line make it 2 line, chop out what excess 42 char
{
nLines = 2;
len = 42; // 2 lines max.
}
if (nLines > 1) // 2 line, now I'm confused???????
{
char buffer[MAX_DMR_ID_CONTACT_TEXT_LENGTH]; // to fit 2 * 21 chars + NULL, but needs larger
memcpy(buffer, text, len + 1);
buffer[len + 1] = 0;
char *p = buffer + 20; // I think it is this line that confused me
// Find a space backward
while ((*p != ' ') && (p > buffer))
{
p--;
}
uint8_t rest = (uint8_t)((buffer + strlen(buffer)) - p) - ((*p == ' ') ? 1 : 0);
// rest is too long, just split the line in two chunks
if (rest > 21)
{
char c = buffer[21];
buffer[21] = 0;
displayPrintCentered(y, chomp(buffer), FONT_SIZE_1); // 2 pixels are saved, could center
buffer[21] = c;
displayPrintCentered(y + 8, chomp(buffer + 21), FONT_SIZE_1);
}
else
{
*p = 0;
displayPrintCentered(y, chomp(buffer), FONT_SIZE_1);
displayPrintCentered(y + 8, chomp(p + 1), FONT_SIZE_1);
}
}
else // One line of 21 chars max
{
displayPrintCentered(y
#if ! defined(PLATFORM_RD5R)
+ 4
#endif
, text, FONT_SIZE_1);
}
}
}
}